feat: salvage-pickup loop — kill-fed charges, scrap drops, salvage-gated card levels

Per canon decisions 2026-08-07 (advisor CONCEPT.md f12cb15, falsifier
response): charges now generate on kills (kills_per_charge, chemistry
chain kills feed the hand; per-hit generation removed). Kills drop
scrap as physical pickups (amber diamond, magnet radius 70, drift to
mech, 20s fade; grunt 1 / burster 3) — collection is the positioning
reward gradient. Cumulative collected scrap gates card levels 1-3
(thresholds 12/30; per-level radius x1.25/x1.5 and power x1.15/x1.5 —
HEAT L3 hits one-cast detonation threshold). HUD: level notches under
slots + scrap counter. Salvage spawn is call_deferred: death can fire
inside a physics signal flush where Area2D setup is blocked.

Smoke gate: charge accrual now kill-fed, salvage drop + collection +
level gating phases added, heat-target selection made retry-tolerant.
4/4 green on nh3-dev headless 4.7.1.

scenes/burster.tscn

@@ -14,6 +14,7 @@ hp = 5.0
 contact_damage = 3.0
 body_radius = 11.0
 base_color = Color(0.6, 0.4, 0.25, 1)
+salvage_value = 3
 self_heat_rate = 0.5
 detonation_damage = 2.5
 detonation_mech_damage = 15.0