feat: tracer bullet — greybox arena, positioning piloting, auto-fire KINETIC weapon

One-screen 640x360 arena (walls, grid floor, fixed view), mech with
positioning-only movement (WASD/arrows, accel/decel, no aim), auto-fire
KINETIC hardpoint (nearest-target, visible range ring) firing slugs
that damage + knock back, untagged grunt trickle spawner with contact
damage, hull bar HUD, scene-reload on hull zero.

Headless smoke gate: tests/smoke.tscn asserts boot, piloting movement,
spawning, input-free auto-fire, grunt lethality, knockback displacement.
3/3 passes on nh3-dev headless 4.7.1.

project.godot

@@ -7,13 +7,52 @@ config_version=5
 [application]
 
 config/name="U R M03"
+run/main_scene="res://scenes/arena.tscn"
 config/features=PackedStringArray("4.7")
 
 [display]
 
+window/size/viewport_width=640
+window/size/viewport_height=360
+window/size/window_width_override=1280
+window/size/window_height_override=720
 window/stretch/mode="viewport"
 window/stretch/scale_mode="integer"
 
+[input]
+
+move_left={
+"deadzone": 0.2,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":65,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+, Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":4194319,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+]
+}
+move_right={
+"deadzone": 0.2,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":68,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+, Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":4194321,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+]
+}
+move_up={
+"deadzone": 0.2,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":87,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+, Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":4194320,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+]
+}
+move_down={
+"deadzone": 0.2,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":83,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+, Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":4194322,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+]
+}
+
+[layer_names]
+
+2d_physics/layer_1="walls"
+2d_physics/layer_2="mech"
+2d_physics/layer_3="enemies"
+2d_physics/layer_4="projectiles"
+
 [rendering]
 
 textures/canvas_textures/default_texture_filter=0

scenes/arena.tscn

@@ -0,0 +1,68 @@
+[gd_scene load_steps=6 format=3]
+
+[ext_resource type="Script" path="res://scripts/arena.gd" id="1"]
+[ext_resource type="PackedScene" path="res://scenes/mech.tscn" id="2"]
+[ext_resource type="Script" path="res://scripts/spawner.gd" id="3"]
+
+[sub_resource type="RectangleShape2D" id="wall_h"]
+size = Vector2(640, 16)
+
+[sub_resource type="RectangleShape2D" id="wall_v"]
+size = Vector2(16, 360)
+
+[node name="Arena" type="Node2D"]
+script = ExtResource("1")
+
+[node name="WallTop" type="StaticBody2D" parent="."]
+position = Vector2(320, 8)
+collision_layer = 1
+collision_mask = 0
+
+[node name="Collision" type="CollisionShape2D" parent="WallTop"]
+shape = SubResource("wall_h")
+
+[node name="WallBottom" type="StaticBody2D" parent="."]
+position = Vector2(320, 352)
+collision_layer = 1
+collision_mask = 0
+
+[node name="Collision" type="CollisionShape2D" parent="WallBottom"]
+shape = SubResource("wall_h")
+
+[node name="WallLeft" type="StaticBody2D" parent="."]
+position = Vector2(8, 180)
+collision_layer = 1
+collision_mask = 0
+
+[node name="Collision" type="CollisionShape2D" parent="WallLeft"]
+shape = SubResource("wall_v")
+
+[node name="WallRight" type="StaticBody2D" parent="."]
+position = Vector2(632, 180)
+collision_layer = 1
+collision_mask = 0
+
+[node name="Collision" type="CollisionShape2D" parent="WallRight"]
+shape = SubResource("wall_v")
+
+[node name="Mech" parent="." instance=ExtResource("2")]
+position = Vector2(320, 180)
+
+[node name="Spawner" type="Node2D" parent="."]
+script = ExtResource("3")
+
+[node name="HUD" type="CanvasLayer" parent="."]
+
+[node name="HullBack" type="ColorRect" parent="HUD"]
+offset_left = 8.0
+offset_top = 8.0
+offset_right = 128.0
+offset_bottom = 14.0
+color = Color(0.06, 0.07, 0.08, 1)
+
+[node name="HullFill" type="ColorRect" parent="HUD"]
+offset_left = 9.0
+offset_top = 9.0
+offset_right = 127.0
+offset_bottom = 13.0
+color = Color(0.55, 0.8, 0.55, 1)

scenes/grunt.tscn

@@ -0,0 +1,14 @@
+[gd_scene load_steps=3 format=3]
+
+[ext_resource type="Script" path="res://scripts/grunt.gd" id="1"]
+
+[sub_resource type="CircleShape2D" id="body"]
+radius = 8.0
+
+[node name="Grunt" type="CharacterBody2D"]
+collision_layer = 4
+collision_mask = 7
+script = ExtResource("1")
+
+[node name="Collision" type="CollisionShape2D" parent="."]
+shape = SubResource("body")

scenes/kinetic_projectile.tscn

@@ -0,0 +1,15 @@
+[gd_scene load_steps=3 format=3]
+
+[ext_resource type="Script" path="res://scripts/kinetic_projectile.gd" id="1"]
+
+[sub_resource type="CircleShape2D" id="body"]
+radius = 4.0
+
+[node name="KineticProjectile" type="Area2D"]
+collision_layer = 8
+collision_mask = 5
+monitorable = false
+script = ExtResource("1")
+
+[node name="Collision" type="CollisionShape2D" parent="."]
+shape = SubResource("body")

scenes/mech.tscn

@@ -0,0 +1,18 @@
+[gd_scene load_steps=4 format=3]
+
+[ext_resource type="Script" path="res://scripts/mech.gd" id="1"]
+[ext_resource type="Script" path="res://scripts/kinetic_weapon.gd" id="2"]
+
+[sub_resource type="RectangleShape2D" id="body"]
+size = Vector2(20, 24)
+
+[node name="Mech" type="CharacterBody2D"]
+collision_layer = 2
+collision_mask = 5
+script = ExtResource("1")
+
+[node name="Collision" type="CollisionShape2D" parent="."]
+shape = SubResource("body")
+
+[node name="Weapon" type="Node2D" parent="."]
+script = ExtResource("2")

scripts/arena.gd

@@ -0,0 +1,31 @@
+extends Node2D
+## One greybox arena, one screen (640x360), fixed camera. Draws its own
+## floor and wall slabs; tracks kills for instrumentation.
+
+var kills: int = 0
+
+
+func _ready() -> void:
+	$Spawner.grunt_spawned.connect(_on_grunt_spawned)
+	$Mech.hull_changed.connect(_on_hull_changed)
+
+
+func _on_grunt_spawned(grunt: Node) -> void:
+	grunt.died.connect(func() -> void: kills += 1)
+
+
+func _on_hull_changed(hull: float, hull_max: float) -> void:
+	$HUD/HullFill.size.x = 118.0 * (hull / hull_max)
+
+
+func _draw() -> void:
+	draw_rect(Rect2(0, 0, 640, 360), Color(0.1, 0.11, 0.13))
+	for x in range(0, 641, 32):
+		draw_line(Vector2(x, 0), Vector2(x, 360), Color(0.13, 0.145, 0.17))
+	for y in range(0, 361, 32):
+		draw_line(Vector2(0, y), Vector2(640, y), Color(0.13, 0.145, 0.17))
+	var wall := Color(0.3, 0.32, 0.36)
+	draw_rect(Rect2(0, 0, 640, 16), wall)
+	draw_rect(Rect2(0, 344, 640, 16), wall)
+	draw_rect(Rect2(0, 0, 16, 360), wall)
+	draw_rect(Rect2(624, 0, 16, 360), wall)

scripts/arena.gd.uid

@@ -0,0 +1 @@
+uid://cw1g4kdg6hn82

scripts/grunt.gd

@@ -0,0 +1,58 @@
+extends CharacterBody2D
+## Untagged grunt — the Act-1 learning-language enemy. Walks at the
+## mech, deals contact damage on a tick. Untagged = neutral hue, no
+## tag behavior. Knockback state lives here so KINETIC (and later
+## chemistry) has something to shove.
+
+signal died
+
+@export var speed := 70.0
+@export var hp := 3.0
+@export var contact_damage := 5.0
+@export var contact_interval := 0.5
+@export var knockback_decay := 5.0
+
+var _knockback := Vector2.ZERO
+var _contact_cd := 0.0
+var _flash := 0.0
+var _mech: Node2D
+
+
+func _ready() -> void:
+	add_to_group("enemies")
+	_mech = get_tree().get_first_node_in_group("mech")
+
+
+func _physics_process(delta: float) -> void:
+	_contact_cd -= delta
+	var seek := Vector2.ZERO
+	if is_instance_valid(_mech):
+		seek = (_mech.global_position - global_position).normalized() * speed
+	_knockback = _knockback.lerp(Vector2.ZERO, 1.0 - exp(-knockback_decay * delta))
+	velocity = seek + _knockback
+	move_and_slide()
+
+	for i in get_slide_collision_count():
+		var collider := get_slide_collision(i).get_collider()
+		if collider == _mech and _contact_cd <= 0.0:
+			_mech.take_damage(contact_damage)
+			_contact_cd = contact_interval
+
+	if _flash > 0.0:
+		_flash -= delta
+		queue_redraw()
+
+
+func hit(damage: float, kb: Vector2) -> void:
+	hp -= damage
+	_knockback += kb
+	_flash = 0.12
+	queue_redraw()
+	if hp <= 0.0:
+		died.emit()
+		queue_free()
+
+
+func _draw() -> void:
+	var body := Color(0.95, 0.95, 0.95) if _flash > 0.0 else Color(0.45, 0.47, 0.4)
+	draw_circle(Vector2.ZERO, 8.0, body)

scripts/grunt.gd.uid

@@ -0,0 +1 @@
+uid://k5sas40w41a6

scripts/kinetic_projectile.gd

@@ -0,0 +1,36 @@
+extends Area2D
+## KINETIC slug: damage plus knockback along its travel direction.
+## Knockback is the load-bearing part — KINETIC is the one tag that
+## touches the piloting layer, and the HEAT+KINETIC chemistry row
+## (next slice) rides on impacts this weapon causes.
+
+@export var speed := 420.0
+@export var damage := 1.0
+@export var knockback := 220.0
+@export var lifetime := 0.8
+
+var direction := Vector2.RIGHT
+
+
+func _ready() -> void:
+	add_to_group("projectiles")
+	rotation = direction.angle()
+	body_entered.connect(_on_body_entered)
+
+
+func _physics_process(delta: float) -> void:
+	position += direction * speed * delta
+	lifetime -= delta
+	if lifetime <= 0.0:
+		queue_free()
+
+
+func _on_body_entered(body: Node) -> void:
+	if body.is_in_group("enemies"):
+		body.hit(damage, direction * knockback)
+	queue_free()
+
+
+func _draw() -> void:
+	# KINETIC hue: pale steel-blue (placeholder — tag colors unassigned in canon).
+	draw_rect(Rect2(-5, -1.5, 10, 3), Color(0.75, 0.85, 0.95))

scripts/kinetic_projectile.gd.uid

@@ -0,0 +1 @@
+uid://ose3uhxglar

scripts/kinetic_weapon.gd

@@ -0,0 +1,44 @@
+extends Node2D
+## Auto-fire KINETIC hardpoint. No player input ever touches this —
+## the build expresses itself while the pilot only positions. Draws its
+## own range ring so the kill zone is legible (positioning <-> build bet).
+
+const PROJECTILE := preload("res://scenes/kinetic_projectile.tscn")
+
+@export var fire_interval := 0.4
+@export var range_px := 220.0
+
+var _cooldown := 0.0
+
+
+func _physics_process(delta: float) -> void:
+	_cooldown -= delta
+	if _cooldown > 0.0:
+		return
+	var target := _nearest_enemy()
+	if target == null:
+		return
+	_fire(target)
+	_cooldown = fire_interval
+
+
+func _nearest_enemy() -> Node2D:
+	var best: Node2D = null
+	var best_d := range_px * range_px
+	for e in get_tree().get_nodes_in_group("enemies"):
+		var d: float = global_position.distance_squared_to(e.global_position)
+		if d < best_d:
+			best_d = d
+			best = e
+	return best
+
+
+func _fire(target: Node2D) -> void:
+	var p := PROJECTILE.instantiate()
+	p.direction = (target.global_position - global_position).normalized()
+	p.position = global_position
+	owner.get_parent().add_child(p)
+
+
+func _draw() -> void:
+	draw_arc(Vector2.ZERO, range_px, 0.0, TAU, 64, Color(0.55, 0.7, 0.85, 0.18), 1.0)

scripts/kinetic_weapon.gd.uid

@@ -0,0 +1 @@
+uid://dn6fsoa45vgu1

scripts/mech.gd

@@ -0,0 +1,37 @@
+class_name Mech
+extends CharacterBody2D
+## M03, greybox slab. Piloting is positioning ONLY (canon: no aim, no
+## combos) — movement input is the entire control surface.
+
+signal hull_changed(hull: float, hull_max: float)
+
+@export var move_speed := 160.0
+@export var accel := 900.0
+@export var decel := 1200.0
+@export var hull_max := 100.0
+
+var hull: float
+
+
+func _ready() -> void:
+	add_to_group("mech")
+	hull = hull_max
+
+
+func _physics_process(delta: float) -> void:
+	var dir := Input.get_vector("move_left", "move_right", "move_up", "move_down")
+	var rate := accel if dir != Vector2.ZERO else decel
+	velocity = velocity.move_toward(dir * move_speed, rate * delta)
+	move_and_slide()
+
+
+func take_damage(amount: float) -> void:
+	hull = maxf(hull - amount, 0.0)
+	hull_changed.emit(hull, hull_max)
+	if hull <= 0.0:
+		get_tree().reload_current_scene()
+
+
+func _draw() -> void:
+	draw_rect(Rect2(-10, -12, 20, 24), Color(0.82, 0.84, 0.88))
+	draw_rect(Rect2(-6, -6, 12, 12), Color(0.55, 0.6, 0.68))

scripts/mech.gd.uid

@@ -0,0 +1 @@
+uid://cyw4tmaglsdu4

scripts/spawner.gd

@@ -0,0 +1,44 @@
+extends Node2D
+## Keeps a floor count of grunts alive, spawning at the arena's inset
+## edges. Endless trickle — the tracer needs targets, not waves.
+
+signal grunt_spawned(grunt: Node)
+
+const GRUNT := preload("res://scenes/grunt.tscn")
+
+@export var target_count := 8
+@export var spawn_interval := 0.8
+@export var spawn_rect := Rect2(32, 32, 576, 296)
+@export var min_dist_from_mech := 120.0
+
+var _timer := 0.0
+
+
+func _physics_process(delta: float) -> void:
+	_timer -= delta
+	if _timer > 0.0:
+		return
+	_timer = spawn_interval
+	if get_tree().get_nodes_in_group("enemies").size() < target_count:
+		_spawn()
+
+
+func _spawn() -> void:
+	var g := GRUNT.instantiate()
+	g.position = _pick_point()
+	add_child(g)
+	grunt_spawned.emit(g)
+
+
+func _pick_point() -> Vector2:
+	var mech := get_tree().get_first_node_in_group("mech") as Node2D
+	var p := Vector2.ZERO
+	for i in 12:
+		match randi() % 4:
+			0: p = Vector2(randf_range(spawn_rect.position.x, spawn_rect.end.x), spawn_rect.position.y)
+			1: p = Vector2(randf_range(spawn_rect.position.x, spawn_rect.end.x), spawn_rect.end.y)
+			2: p = Vector2(spawn_rect.position.x, randf_range(spawn_rect.position.y, spawn_rect.end.y))
+			3: p = Vector2(spawn_rect.end.x, randf_range(spawn_rect.position.y, spawn_rect.end.y))
+		if mech == null or p.distance_to(mech.global_position) >= min_dist_from_mech:
+			return p
+	return p

scripts/spawner.gd.uid

@@ -0,0 +1 @@
+uid://bo1kuwuh0xjyb

tests/smoke.gd

@@ -0,0 +1,95 @@
+extends Node
+## Headless smoke test for the tracer bullet. Run:
+##   godot --headless --path . res://tests/smoke.tscn
+## Asserts: arena boots, piloting moves the mech, grunts spawn, the
+## weapon auto-fires with no input, a grunt dies, knockback displaces.
+## Exit 0 = pass, 1 = fail.
+
+const ARENA := preload("res://scenes/arena.tscn")
+
+var _failures: Array[String] = []
+var _done := false
+
+
+func _ready() -> void:
+	get_tree().create_timer(25.0).timeout.connect(_on_global_timeout)
+	await _run()
+	_finish()
+
+
+func _on_global_timeout() -> void:
+	if not _done:
+		_failures.append("global 25s timeout — test hung")
+		_finish()
+
+
+func _run() -> void:
+	var arena := ARENA.instantiate()
+	add_child(arena)
+	await get_tree().physics_frame
+	await get_tree().physics_frame
+
+	var mech := arena.get_node_or_null("Mech") as CharacterBody2D
+	if mech == null:
+		_failures.append("no Mech node in arena")
+		return
+
+	# Piloting: press right, mech must move right.
+	var x0 := mech.global_position.x
+	Input.action_press("move_right")
+	for i in 30:
+		await get_tree().physics_frame
+	Input.action_release("move_right")
+	if mech.global_position.x <= x0 + 10.0:
+		_failures.append("piloting: mech did not move right (x %.1f -> %.1f)" % [x0, mech.global_position.x])
+
+	# Spawner: grunts appear.
+	if not await _until(func() -> bool: return get_tree().get_nodes_in_group("enemies").size() > 0, 5.0, "no grunts spawned"):
+		return
+
+	# Auto-fire: a projectile appears with zero input.
+	await _until(func() -> bool: return get_tree().get_nodes_in_group("projectiles").size() > 0, 5.0, "weapon never auto-fired")
+
+	# Lethality: at least one grunt dies to auto-fire alone.
+	await _until(func() -> bool: return arena.kills >= 1, 10.0, "no grunt died to auto-fire")
+
+	# Knockback: a zero-damage hit must displace a grunt along the hit
+	# direction. Probe the healthiest grunt — one mid-death from
+	# auto-fire would vanish under the probe and fake a failure.
+	var g: CharacterBody2D = null
+	for cand in get_tree().get_nodes_in_group("enemies"):
+		if g == null or cand.hp > g.hp:
+			g = cand
+	if g != null:
+		var gx := g.global_position.x
+		g.hit(0.0, Vector2(400, 0))
+		for i in 5:
+			await get_tree().physics_frame
+		if not is_instance_valid(g):
+			_failures.append("knockback probe grunt died to a zero-damage hit")
+		elif g.global_position.x <= gx + 2.0:
+			_failures.append("knockback: grunt did not displace (x %.1f -> %.1f)" % [gx, g.global_position.x])
+
+
+func _until(pred: Callable, timeout: float, what: String) -> bool:
+	var t := 0.0
+	while t < timeout:
+		if pred.call():
+			return true
+		await get_tree().physics_frame
+		t += get_physics_process_delta_time()
+	_failures.append("timeout: " + what)
+	return false
+
+
+func _finish() -> void:
+	if _done:
+		return
+	_done = true
+	if _failures.is_empty():
+		print("SMOKE PASS")
+		get_tree().quit(0)
+	else:
+		for f in _failures:
+			printerr("SMOKE FAIL: " + f)
+		get_tree().quit(1)

tests/smoke.gd.uid

@@ -0,0 +1 @@
+uid://qcvs3eu210cv

tests/smoke.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://tests/smoke.gd" id="1"]
+
+[node name="Smoke" type="Node"]
+script = ExtResource("1")