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>

context/open-questions.md

@@ -480,6 +480,25 @@ register.  ⚠ The audit also flags the damage-economy item as SELF-CONTRADICTOR
 - The WinSock2 stack (L4NET.CPP) is 2-node verified end-to-end (cross-pod kills, beams,
   replicant gait — tasks #46-#51); remaining = the real pod-LAN config (real IPs, bare-IP
   pilot entries).
+- **✅ Subsystem-record replication IS wired — the AC just wasn't using it (task #61,
+  2026-07-13) [T2, CORRECTED].** (An earlier draft of this note WRONGLY claimed "no
+  subsystem-record channel" — that was concluded from an AC-only test before the AC's
+  ForceUpdate gap was found.  The channel exists and works.)  MECHANISM: the mech's per-frame
+  tick calls `subsystem->PerformAndWatch(till, update_stream)` for every subsystem
+  (mech4.cpp:3893) → base `Simulation::WriteSimulationUpdate` walks the subsystem's `updateModel`
+  and serializes any set record; `Entity::UpdateMessageHandler` (ENTITY.cpp:387) routes each
+  incoming record to `GetSimulation(subsystemID-1)->ReadUpdateRecord` on the peer.  A subsystem
+  replicates its fire IFF its `FireWeapon` calls **`ForceUpdate()`** (sets the updateModel bit).
+  The EMITTER (beam) and MISSILELAUNCHER (salvo mirror, mislanch.cpp:326) both do → enemy lasers
+  + missiles ARE visible on the peer.  The AUTOCANNON set only `simulationFlags |= 0x1` (the
+  +0x28 instance flag, NOT updateModel) and had no fire record → its shot never serialized → the
+  enemy's cannon (tracer + DAFC muzzle flash) was invisible.  FIX: added
+  `ProjectileWeapon::WriteUpdateRecord/ReadUpdateRecord` (fire counter + aim, the AC twin of the
+  salvo mirror) + `ForceUpdate()` in FireWeapon.  Verified live: watching node now logs
+  REPLICANT AC shots + muzzle flashes.  **LESSON (methodology):** the binary-coverage audit is a
+  function-PRESENCE census — it can't see "reconstructed but inert" code (a function that exists
+  but is never CALLED, like the salvo mirror was thought to be, or a missing ForceUpdate).  A
+  LIVENESS audit (does a live call path reach each reconstructed fn?) would catch this class.
 
 ## Rendering follow-ups (non-blocking)
 - **.PFX effect-layer polish (deferred 2026-07-08; the layer itself is LIVE + verified).** The