Ballistic FX: DAFC muzzle flash + autocannon fire replication (task #61)

The autocannon gains its authentic muzzle effect AND becomes visible when an
enemy fires it -- the ballistic-FX gap behind "I only see the AC from my own
view."

MUZZLE FLASH (the genuine shipped effect, not the cut card):
- MUZFLASH.BGF is an orphaned/cut asset (nothing references it) -- so it is NOT
  rendered.  The shipped projectile-gun muzzle effect is DAFC.PFX, which
  BTDPL.INI documents as "the effect used on all projectile guns" (psfx 6
  external / 14 internal): an orange fire-smoke blast (btfx:firesmoke1),
  maxIssue 25 over ~0.2s so the emitter auto-expires = one burst per shot.
- BTFlashMuzzle (mech4.cpp) spawns it on the gun-port SEGMENT via the existing
  BTStartPfxAttached path; the segment frame sprays -Z out the barrel.  Hooked
  at ProjectileWeapon::FireWeapon (the fire edge).  Default ON (BT_MUZZLE=0
  disables).  AC only -- lasers show their beam, missiles their launch.

ENEMY AC FIRE NOW REPLICATES (the real find):
- ROOT CAUSE: the subsystem-record replication channel EXISTS and works (mech
  ticks subsystem->PerformAndWatch(update_stream); Entity::UpdateMessageHandler
  routes incoming records to GetSimulation(subsystemID-1)->ReadUpdateRecord).
  The emitter (beam) and MissileLauncher (salvo mirror) both call ForceUpdate()
  so their fire serializes -> enemy lasers + missiles ARE visible on the peer.
  The AUTOCANNON set only `simulationFlags |= 0x1` (the +0x28 instance flag,
  NOT the updateModel bit WriteSimulationUpdate walks) and had NO fire record,
  so its shot never crossed the wire -- the enemy's cannon was invisible.
- FIX (the AC twin of the missile salvo mirror): ProjectileWeapon::
  WriteUpdateRecord/ReadUpdateRecord (fire counter + aim) + ForceUpdate() in
  FireWeapon.  The replicant edge-detects the counter and mirrors ONE visual
  round + DAFC muzzle flash from its own resolved muzzle; a null/untargeted aim
  streaks straight out the barrel (launchVelocity) instead of toward origin.
- Verified live 2-node: the watching node logs REPLICANT AC shots + DAFC
  flashes at the enemy's gun-port (seg 7), aimed shots fly to the real target;
  no crashes.

TRACER: all ACs author TracerInterval=1 (every round is a tracer); the existing
amber streak is acceptable-authentic, so no tracer change was needed.

KB: open-questions.md -- CORRECTED the (wrong) earlier note that claimed no
subsystem-record channel exists; it does, the AC just wasn't using it.  Logged
the methodology lesson: the coverage audit finds UNWRITTEN functions, not
"reconstructed but inert" ones (a function present but never called -- the AC
record + the missile mirror both looked done); a LIVENESS audit would catch
that class.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

game/reconstructed/mech4.cpp

@@ -851,6 +851,40 @@ int
 	return 1;
 }
 
+//###########################################################################
+// MUZZLE FLASH (task #61) -- the GENUINE shipped projectile-gun muzzle effect.
+//
+// BTDPL.INI documents psfx 6 (external) / 14 (internal) = DAFC.PFX as "the
+// effect used on all projectile guns": an orange fire-smoke blast
+// (btfx:firesmoke1, maxIssue 25 / releasePeriod ~0.2s -> the emitter auto-
+// expires after one ~0.2s burst = a per-shot muzzle flash).  The AC is the
+// only weapon that gets it -- lasers show their beam, missiles their launch.
+// NOT the cut MUZFLASH.BGF card (that asset is orphaned; the shipped flash is
+// this particle).  Disasm of ProjectileWeapon::FireWeapon (@0x4bc104) shows no
+// direct spawn there, so the fire edge is the faithful hook; the effect,
+// asset, and gun-port attach point are all confirmed shipped [T1 asset/INI].
+//
+// Attached to the gun-port SEGMENT (BTResolveSegmentWorld re-resolves the
+// muzzle position + mech body frame each frame): DAFC sprays -Z, and the mech
+// faces -Z, so the fire-smoke blows forward out the barrel.
+//###########################################################################
+void
+	BTFlashMuzzle(void *ownerMech, int seg_index, float mx, float my, float mz)
+{
+	if (ownerMech == 0)
+		return;
+	float pos[3], rows[9];
+	if (!BTResolveSegmentWorld(ownerMech, seg_index, pos, rows))
+		return;
+	extern void BTStartPfxAttached(int, void *, int, float, float, float, const float *);
+	// psfx 6 = DAFC (external); the day table maps both 6 and 14 to DAFC.PFX,
+	// so the external slot serves every viewer.
+	BTStartPfxAttached(6, ownerMech, seg_index, mx, my, mz, rows);
+	if (getenv("BT_MUZZLE_LOG"))
+		DEBUG_STREAM << "[muzzle] DAFC flash seg=" << seg_index
+			<< " at(" << mx << "," << my << "," << mz << ")\n" << std::flush;
+}
+
 //###########################################################################
 // PER-ROUND DETONATION (the binary's Missile::MoveAndCollide @004bef78: every
 // round spawns ITS OWN ExplosionModelFile at its impact point, resource