feat: KINETIC+HEAT chemistry row + self-heating burster — greybox build complete

Chemistry row *Spread* (canon CONCEPT.md § tag grammar): a heated
enemy slammed into another (knockback impact >= threshold, not a
walking bump) spreads its heat on contact and passes momentum, so
chains cascade with decaying force — chemistry feeding chemistry.
Authorship-loud feedback stack per canon: ~80ms hitstop (owned by the
hand, capped so chain procs can't lock the world; rides on top of
freeze/thaw states), the two tag hues meeting at the point of impact
(oriented fx, no proc text), and a per-pair audio sting (procedural
placeholder, _ph-marked per asset policy).

Burster: second enemy type, HEAT-tagged walking bomb (orange base,
literal tag mirror) — self-heats ~0.5/s toward threshold, big
detonation. Every 4th spawn. Detonations are now symmetric: they
damage enemies AND the mech in radius (one grammar on both sides;
heat play prices standing close). Grunt scene exports drive both
enemy types — no subclass.

Smoke gate extended: burster self-heat observed (retry-tolerant),
deterministic staged chemistry proc (heated striker slammed into cold
neighbor outside weapon range -> proc counter, heat spread). 3/3
green on nh3-dev headless 4.7.1. Known cosmetic: 2 ObjectDB instances
reported leaked at exit (constant, exit-order artifact — audio stream
alive at quit), not a runtime leak.

assets/audio/chem_sting_kinetic_heat_ph.wav.import

@@ -0,0 +1,24 @@
+[remap]
+
+importer="wav"
+type="AudioStreamWAV"
+uid="uid://c6t7uwmnlqaup"
+path="res://.godot/imported/chem_sting_kinetic_heat_ph.wav-8ea5181b52abc6efbaead61e1d2c7753.sample"
+
+[deps]
+
+source_file="res://assets/audio/chem_sting_kinetic_heat_ph.wav"
+dest_files=["res://.godot/imported/chem_sting_kinetic_heat_ph.wav-8ea5181b52abc6efbaead61e1d2c7753.sample"]
+
+[params]
+
+force/8_bit=false
+force/mono=false
+force/max_rate=false
+force/max_rate_hz=44100
+edit/trim=false
+edit/normalize=false
+edit/loop_mode=0
+edit/loop_begin=0
+edit/loop_end=-1
+compress/mode=2

scenes/arena.tscn

@@ -1,10 +1,11 @@
-[gd_scene load_steps=8 format=3]
+[gd_scene load_steps=9 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"]
 [ext_resource type="Script" path="res://scripts/hand.gd" id="4"]
 [ext_resource type="Script" path="res://scripts/hand_hud.gd" id="5"]
+[ext_resource type="AudioStream" path="res://assets/audio/chem_sting_kinetic_heat_ph.wav" id="6"]
 
 [sub_resource type="RectangleShape2D" id="wall_h"]
 size = Vector2(640, 16)
@@ -53,6 +54,10 @@ position = Vector2(320, 180)
 [node name="Spawner" type="Node2D" parent="."]
 script = ExtResource("3")
 
+[node name="Sting" type="AudioStreamPlayer" parent="."]
+stream = ExtResource("6")
+volume_db = -6.0
+
 [node name="HUD" type="CanvasLayer" parent="."]
 
 [node name="HullBack" type="ColorRect" parent="HUD"]

scenes/burster.tscn

@@ -0,0 +1,23 @@
+[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 = 11.0
+
+[node name="Burster" type="CharacterBody2D"]
+collision_layer = 4
+collision_mask = 7
+script = ExtResource("1")
+speed = 45.0
+hp = 5.0
+contact_damage = 3.0
+body_radius = 11.0
+base_color = Color(0.6, 0.4, 0.25, 1)
+self_heat_rate = 0.5
+detonation_damage = 2.5
+detonation_mech_damage = 15.0
+detonation_radius = 80.0
+
+[node name="Collision" type="CollisionShape2D" parent="."]
+shape = SubResource("body")

scripts/arena.gd

@@ -1,15 +1,34 @@
 extends Node2D
 ## One greybox arena, one screen (640x360), fixed camera. Draws its own
-## floor and wall slabs; tracks kills for instrumentation.
+## floor and wall slabs; tracks kills and chemistry procs; conducts the
+## authorship-loud chemistry feedback stack (canon: brief hitstop + the
+## two tag hues meeting at the point of contact + a per-pair sting —
+## no proc text, the light and the sting are the noun).
+
+const CHEM_FX := preload("res://scripts/chemistry_fx.gd")
 
 var kills: int = 0
+var chem_procs: int = 0
 
 
 func _ready() -> void:
+	add_to_group("arena")
 	$Spawner.grunt_spawned.connect(_on_grunt_spawned)
 	$Mech.hull_changed.connect(_on_hull_changed)
 
 
+func chemistry_proc(at: Vector2, axis: Vector2) -> void:
+	chem_procs += 1
+	var hand := get_tree().get_first_node_in_group("hand")
+	if hand != null:
+		hand.hitstop()
+	var fx: Node2D = CHEM_FX.new()
+	fx.position = at
+	fx.axis = axis
+	add_child(fx)
+	$Sting.play()
+
+
 func _on_grunt_spawned(grunt: Node) -> void:
 	grunt.died.connect(func() -> void: kills += 1)
 

scripts/chemistry_fx.gd

@@ -0,0 +1,29 @@
+extends Node2D
+## KINETIC+HEAT chemistry proc marker: the two tag hues visibly meet
+## at the point of impact (canon feedback stack — the light and the
+## sting are the noun; no proc text, ever). Oriented along the impact
+## axis: KINETIC steel-blue sweeps in from the striker side, HEAT
+## orange flares out the struck side, white core where they touch.
+
+var axis := Vector2.RIGHT
+var duration := 0.28
+var max_r := 26.0
+
+var _t := 0.0
+
+
+func _process(delta: float) -> void:
+	_t += delta
+	queue_redraw()
+	if _t >= duration:
+		queue_free()
+
+
+func _draw() -> void:
+	var k := clampf(_t / duration, 0.0, 1.0)
+	var a := 1.0 - k
+	var r := lerpf(6.0, max_r, k)
+	var ang := axis.angle()
+	draw_arc(Vector2.ZERO, r, ang + PI - 1.2, ang + PI + 1.2, 20, Color(0.75, 0.85, 0.95, a), 3.0)
+	draw_arc(Vector2.ZERO, r * 0.8, ang - 1.2, ang + 1.2, 20, Color(0.95, 0.55, 0.2, a), 3.0)
+	draw_circle(Vector2.ZERO, lerpf(5.0, 1.0, k), Color(1, 1, 1, a))

scripts/chemistry_fx.gd.uid

@@ -0,0 +1 @@
+uid://55wtetewo5wd

scripts/fx_ring.gd.uid

@@ -0,0 +1 @@
+uid://uq5254gd1260

scripts/grunt.gd

@@ -1,13 +1,22 @@
 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 of its own; heat is a status OTHERS put on it.
+## Enemy body — used by both greybox enemy types via scene exports:
+## the untagged grunt (neutral hue, no tag behavior of its own) and
+## the self-heating burster (self_heat_rate > 0: HEAT-tagged walking
+## bomb, orange base per the one-color legibility floor — the literal
+## tag mirror made playable: its own mechanic is your ammo if you
+## knock it into the swarm before it pops).
 ##
 ## Heat: accumulates, glows, detonates at threshold (HEAT's signature).
-## The threshold check runs in physics so detonations resolve when time
-## flows — stack heat while frozen, release, pop. Detonation is damage
-## only, zero knockback: moving things is KINETIC's verb, not HEAT's
-## (tag purity keeps the chemistry legible).
+## Detonation is symmetric — it damages enemies AND the mech in radius
+## (one grammar on both sides; your heat play prices standing close).
+## Detonation deals no knockback: moving things is KINETIC's verb.
+##
+## Chemistry row KINETIC+HEAT (*Spread*): a heated enemy slammed into
+## another (knockback impact, not a walking bump) spreads its heat on
+## contact, passes momentum, and fires the authorship-loud feedback
+## stack via the arena. Struck enemies are heated AND shoved, so
+## chains can cascade — chemistry feeding chemistry, decaying with
+## each hop.
 
 signal died
 
@@ -16,17 +25,31 @@ signal died
 @export var contact_damage := 5.0
 @export var contact_interval := 0.5
 @export var knockback_decay := 5.0
+@export var body_radius := 8.0
+@export var base_color := Color(0.45, 0.47, 0.4)
+## > 0 makes this a burster: heat/second it applies to itself.
+@export var self_heat_rate := 0.0
 
 @export_group("Heat status")
 @export var heat_threshold := 3.0
 @export var detonation_damage := 1.5
+@export var detonation_mech_damage := 6.0
 @export var detonation_radius := 56.0
 
+@export_group("Chemistry KINETIC+HEAT")
+## Minimum knockback speed for an impact to count as "knocked into".
+@export var chem_min_knockback := 120.0
+## Fraction of the striker's heat the struck enemy receives.
+@export var chem_spread_factor := 1.0
+## Fraction of the striker's knockback passed to the struck enemy.
+@export var chem_momentum_factor := 0.45
+
 const FX_RING := preload("res://scripts/fx_ring.gd")
 
 var heat := 0.0
 var _knockback := Vector2.ZERO
 var _contact_cd := 0.0
+var _chem_cd := 0.0
 var _flash := 0.0
 var _dead := false
 var _mech: Node2D
@@ -34,15 +57,21 @@ var _mech: Node2D
 
 func _ready() -> void:
 	add_to_group("enemies")
+	if self_heat_rate > 0.0:
+		add_to_group("bursters")
 	_mech = get_tree().get_first_node_in_group("mech")
 
 
 func _physics_process(delta: float) -> void:
+	if self_heat_rate > 0.0:
+		heat += self_heat_rate * delta
+		queue_redraw()
 	if heat >= heat_threshold:
 		_detonate()
 		return
 
 	_contact_cd -= delta
+	_chem_cd -= delta
 	var seek := Vector2.ZERO
 	if is_instance_valid(_mech):
 		seek = (_mech.global_position - global_position).normalized() * speed
@@ -51,10 +80,21 @@ func _physics_process(delta: float) -> void:
 	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
+		var col := get_slide_collision(i)
+		var collider := col.get_collider()
+		if collider == _mech:
+			if _contact_cd <= 0.0:
+				_mech.take_damage(contact_damage)
+				_contact_cd = contact_interval
+		elif collider is CharacterBody2D and collider.is_in_group("enemies"):
+			if heat > 0.0 and _chem_cd <= 0.0 and _knockback.length() >= chem_min_knockback:
+				_chem_cd = 0.12
+				collider.add_heat(heat * chem_spread_factor)
+				collider.hit(0.0, _knockback * chem_momentum_factor)
+				var arena := get_tree().get_first_node_in_group("arena")
+				if arena != null:
+					var axis: Vector2 = (collider.global_position - global_position).normalized()
+					arena.chemistry_proc(col.get_position(), axis)
 
 	if _flash > 0.0:
 		_flash -= delta
@@ -87,6 +127,8 @@ func _detonate() -> void:
 			continue
 		if global_position.distance_to(e.global_position) <= detonation_radius:
 			e.hit(detonation_damage, Vector2.ZERO)
+	if is_instance_valid(_mech) and global_position.distance_to(_mech.global_position) <= detonation_radius:
+		_mech.take_damage(detonation_mech_damage)
 	var ring: Node2D = FX_RING.new()
 	ring.position = global_position
 	ring.max_radius = detonation_radius
@@ -103,12 +145,12 @@ func _die() -> void:
 
 
 func _draw() -> void:
-	var body := Color(0.45, 0.47, 0.4)
+	var body := base_color
 	var heat_k := clampf(heat / heat_threshold, 0.0, 1.0)
 	if heat_k > 0.0:
 		body = body.lerp(Color(0.95, 0.5, 0.15), heat_k * 0.8)
 	if _flash > 0.0:
 		body = Color(0.95, 0.95, 0.95)
-	draw_circle(Vector2.ZERO, 8.0, body)
+	draw_circle(Vector2.ZERO, body_radius, body)
 	if heat_k > 0.0:
-		draw_arc(Vector2.ZERO, 10.0, 0.0, TAU, 24, Color(0.95, 0.55, 0.2, 0.4 + heat_k * 0.5), 1.5)
+		draw_arc(Vector2.ZERO, body_radius + 2.0, 0.0, TAU, 24, Color(0.95, 0.55, 0.2, 0.4 + heat_k * 0.5), 1.5)

scripts/hand.gd

@@ -57,6 +57,9 @@ extends CanvasLayer
 ## In-band thaw acceleration: 1 = linear; higher hangs slow longer,
 ## then rushes to full speed.
 @export var thaw_curve := 2.2
+## Chemistry-proc hitstop length in real seconds (the hand owns time,
+## so it owns the stop; capped so chain procs can't lock the world).
+@export var hitstop_s := 0.08
 
 @export_group("KINETIC card")
 @export var kinetic_cost := 1
@@ -78,6 +81,7 @@ var _hit_progress := 0
 var _last_ms := 0
 var _release_from := 1.0
 var _release_t := -1.0
+var _hitstop_t := 0.0
 
 # Slot order is the hand: Q = slot 1, E = slot 2.
 var cards: Array[Dictionary] = []
@@ -140,6 +144,15 @@ func _process(_delta: float) -> void:
 		Engine.time_scale = lerpf(_release_from, 1.0, k)
 		if k >= 1.0:
 			_release_t = -1.0
+
+	# Hitstop rides on top of whatever time state is current (it can
+	# only slow, never speed up); expiry re-derives the proper state.
+	if _hitstop_t > 0.0:
+		_hitstop_t -= real_dt
+		if not get_tree().paused:
+			Engine.time_scale = minf(Engine.time_scale, 0.05)
+		if _hitstop_t <= 0.0:
+			_apply_time_state()
 	$HUD.queue_redraw()
 
 
@@ -164,6 +177,10 @@ func on_build_hit() -> void:
 		charges = mini(charges + 1, max_charges)
 
 
+func hitstop(dur := -1.0) -> void:
+	_hitstop_t = minf(maxf(_hitstop_t, dur if dur > 0.0 else hitstop_s), 0.15)
+
+
 func can_open() -> bool:
 	return not meter_mode or meter / meter_max >= band_unavailable
 

scripts/hand.gd.uid

@@ -0,0 +1 @@
+uid://v8l1rui4tw4i

scripts/hand_hud.gd.uid

@@ -0,0 +1 @@
+uid://badphx2miqxlc

scripts/spawner.gd

@@ -5,13 +5,17 @@ extends Node2D
 signal grunt_spawned(grunt: Node)
 
 const GRUNT := preload("res://scenes/grunt.tscn")
+const BURSTER := preload("res://scenes/burster.tscn")
 
 @export var target_count := 8
 @export var spawn_interval := 0.8
+## Every Nth spawn is a burster.
+@export var burster_every := 4
 @export var spawn_rect := Rect2(32, 32, 576, 296)
 @export var min_dist_from_mech := 120.0
 
 var _timer := 0.0
+var _spawned := 0
 
 
 func _physics_process(delta: float) -> void:
@@ -24,7 +28,9 @@ func _physics_process(delta: float) -> void:
 
 
 func _spawn() -> void:
-	var g := GRUNT.instantiate()
+	_spawned += 1
+	var scene := BURSTER if _spawned % burster_every == 0 else GRUNT
+	var g := scene.instantiate()
 	g.position = _pick_point()
 	add_child(g)
 	grunt_spawned.emit(g)

tests/smoke.gd

@@ -140,6 +140,7 @@ func _test_hand(arena: Node, mech: CharacterBody2D) -> void:
 		_failures.append("heated grunt failed to detonate after release (heat %.1f)" % target.heat)
 
 	await _test_meter(hand)
+	await _test_chemistry(arena, mech)
 
 
 func _test_meter(hand: Node) -> void:
@@ -194,6 +195,52 @@ func _test_meter(hand: Node) -> void:
 	await _until(func() -> bool: return Engine.time_scale >= 0.999, 3.0, "toggle-mode release ramp never reached full speed")
 
 
+func _test_chemistry(arena: Node, mech: CharacterBody2D) -> void:
+	# Burster: a self-heating enemy spawns and its heat grows on its own.
+	# It may die to auto-fire or pop mid-observation, so retry a few.
+	var observed_growth := false
+	for attempt in 4:
+		if not await _until(func() -> bool: return get_tree().get_first_node_in_group("bursters") != null, 20.0, "no burster ever spawned"):
+			return
+		var b: Node2D = get_tree().get_first_node_in_group("bursters")
+		var h0: float = b.heat
+		for i in 30:
+			await get_tree().physics_frame
+		if is_instance_valid(b):
+			if b.heat > h0:
+				observed_growth = true
+				break
+			_failures.append("burster heat did not grow (%.2f -> %.2f)" % [h0, b.heat])
+			return
+	if not observed_growth:
+		_failures.append("never observed a burster long enough to confirm self-heat")
+		return
+
+	# Chemistry row KINETIC+HEAT, deterministic: stage a heated striker
+	# and a cold neighbor in the far corner (outside weapon range),
+	# slam the striker into the neighbor, expect the proc: heat spread,
+	# momentum passed, arena proc counter up.
+	var plain: Array[Node2D] = []
+	for e in get_tree().get_nodes_in_group("enemies"):
+		if e.self_heat_rate == 0.0 and not e._dead:
+			plain.append(e)
+	if plain.size() < 2:
+		_failures.append("not enough plain grunts alive to stage chemistry")
+		return
+	var striker := plain[0]
+	var struck := plain[1]
+	striker.global_position = Vector2(80.0, 80.0)
+	struck.global_position = Vector2(104.0, 80.0)
+	striker.heat = 1.0
+	struck.heat = 0.0
+	var procs0: int = arena.chem_procs
+	striker.hit(0.0, Vector2(500.0, 0.0))
+	if not await _until(func() -> bool: return arena.chem_procs > procs0, 5.0, "chemistry proc never fired on knocked-heated impact"):
+		return
+	if not is_instance_valid(struck) or struck.heat <= 0.0:
+		_failures.append("chemistry proc fired but heat did not spread to the struck enemy")
+
+
 func _nearest_grunt(mech: CharacterBody2D) -> Node2D:
 	var best: Node2D = null
 	var best_d := 120.0 * 120.0