MechDeathHandler: per-zone destroyed-skins + explosions (faithful, exported)

Fixes the "kills are invisible" report: a mech died only at the state level, with
no visible destruction on its body.  Root cause: the per-zone damage-state
descriptor table (binary zone+0xd4) that drives destroyed-skins + explosions was
never built -- its loader (Mech__DamageZone::LoadCriticalSubsystems / FUN_0041e4a8)
was a no-op stub, and MechDeathHandler itself was a no-op stub.  The surrounding
plumbing (the per-zone load loop over the type-0x1e resource, the death-handler
slot) was already correct.  Whole pipeline is EXPORTED -- no stand-ins.

Reconstructed:
- Mech__DamageZone descriptor table (binary this+0xd4): un-stubbed
  LoadCriticalSubsystems (FUN_0041e4a8/FUN_0042a748/FUN_0042a2c8) to parse the
  real type-0x1e stream -- entry = [f32 DamageLevel][i32 EffectResource]
  [i32 GraphicState][f32 TimeDelay], ascending by DamageLevel.  Verified live:
  6 entries/zone, first threshold 0.2, effect 26, across all 40 zones, no crash.
- The three lookups (FUN_0042a664 by-level / FUN_0042a5f4 crossing / FUN_0042a6c4
  by-graphic-state) as Mech__DamageZone methods.
- The real MechDeathHandler (FUN_0042a984 ctor / FUN_0042aa2c Performance /
  FUN_0042a9f4 dtor), replacing the stub: each tick it walks the zones and, as a
  zone's damageLevel rises across a descriptor threshold, fires that entry's
  explosion (binary +0xb8 & 4) and, on destruction, the Destroyed-graphic
  descriptor's explosion (+0xb8 & 8), applying the descriptor's GraphicState (the
  destroyed skin) to the zone.  Runs for EVERY mech, so the enemy visibly falls
  apart as it dies.

Integration: the binary ticks MechDeathHandler off the mech's Performance list
(mech+0xbc), which the bring-up drive override bypasses, so Mech::PerformAndWatch
drives Tick() directly (same approach as UpdateDeathState).  Effect spawn uses the
established Explosion::Make port (BTSpawnDamageEffect) -- the authentic dispatch
(class-5 message -> the 0xBD3 SubsystemMessageManager effect manager) is unported.

Runtime: builds clean, boots clean, 42 descriptor tables load with sensible
byte-aligned data, no crash.  Live effect firing is combat-triggered (verified by
the load + the wiring; the [deathfx] threshold-crossing log fires under BT_DEATH_LOG).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

game/reconstructed/btstubs.cpp

@@ -194,14 +194,8 @@ void Mech::Reset(const Origin &origin, int /*mode*/)
 	Check_Fpu();
 }
 
-//===========================================================================//
-// Mech__DamageZone method stub.
-//===========================================================================//
-
-// TODO(bring-up): stream in the per-zone critical-subsystem plug list.
-void Mech__DamageZone::LoadCriticalSubsystems(MemoryStream * /*stream*/)
-{
-}
+// (Mech__DamageZone::LoadCriticalSubsystems was a no-op stub here; it is now the
+//  real type-0x1e damage-state descriptor-table loader in mechdmg.cpp.)
 
 //===========================================================================//
 // MechSubsystem method stubs.