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.

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")