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/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")