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.

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"]