MP task #47 SOLVED: cross-pod damage + kill (validate the master mech)

Full cross-pod KILL now works: instance A (host 2) shoots B's mech (host 3)
dead over the network; B runs the death transition on its own screen. Verified
on a 2-node one-box run (BT_MP_FORCE_DMG on A): B's mech takes ~36 remote hits
of 12 and is DESTROYED, with wreck swap + explosion.

ROOT CAUSE (one bug): B's own MASTER mech was INVALID. Entity::Receive(Event*)
does `if(!IsValid()) event->Defer()` -- so a cross-pod-delivered TakeDamage that
arrived, resolved to B's real mech, and Posted, was DEFERRED forever; the handler
never ran. The reconstructed Mech::Make force-set ValidFlag only for replicants
(the P6 bring-up hack for the partial MakeReady handshake); a locally-created
master never got validated.

FIX (one line): Mech::Make now sets ValidFlag for the master too -- the
reconstructed ctor builds the mech synchronously, so it's safe. Nothing else
changed: targeting, the virtual victim->Dispatch, the replicant reroute, the
wire, receive, and id resolution were ALL already correct.

Two prior root causes were BOTH mis-diagnoses from the WRONG messageID:
Entity::TakeDamageMessageID is 18, not 21. Every earlier probe filtered
msgID==21, never matched a real TakeDamage, and mis-correlated the stray 21
hits' 3:19 / classID-48 against the real 3:22 hit -- inventing (a) a "cross-TU
Entity layout divergence" (disproven: offsetof identical in both TUs, sizeof
444, entityID@0x17C) and (b) a "localID corrupted on the wire". Both FALSE; the
id is correct end-to-end (3:22 = B's master).

- game/reconstructed/mech.cpp: validate master + replicant at Make.
- game/reconstructed/mech4.cpp: BT_MP_FORCE_DMG cleaned to a concise gated
  2-node cross-pod reproducer (real virtual Dispatch path).
- context/multiplayer.md, reconstruction-gotchas.md: corrected root cause +
  the validity-defers-cross-pod gotcha. Engine diagnostics reverted to clean.

Solo un-regressed (the solo enemy was already force-valid; the change only ADDS
ValidFlag). btl4.exe builds; full mission loop clean.

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

game/reconstructed/mech4.cpp

@@ -2409,16 +2409,13 @@ void
 						<< " zones=" << m->damageZoneCount << "\n" << std::flush;
 				}
 
-			// CROSS-POD DAMAGE PROOF HOOK (BT_MP_FORCE_DMG, task #46/#47): once a
-			// second, dispatch a real unaimed TakeDamage at the first live
-			// REPLICANT candidate through the SAME engine path a beam hit uses
-			// (Entity::Dispatch -> replicant reroute -> SendMessage(ownerID)).
-			// This isolates the cross-pod delivery from the interactive aim/drive
-			// plumbing so a 2-node test can watch the wire without having to line
-			// up a boresight.  It does NOT stamp/patch the id -- Entity::Dispatch
-			// reads this->entityID at offset 0x17C, which the [mp-layout] probe
-			// proved is IDENTICAL in the reconstructed and engine TUs (both 380,
-			// sizeof(Entity)=444), so no cross-TU translation is needed or done.
+			// CROSS-POD DAMAGE TEST HOOK (BT_MP_FORCE_DMG, task #47): once a second,
+			// dispatch an unaimed TakeDamage at the first live REPLICANT through the
+			// SAME virtual Entity::Dispatch path a beam hit uses -- Entity::Dispatch
+			// reroutes a replicant's message to the owning master over the wire, the
+			// master's now-valid mech resolves + applies it (cylinder lookup).  Lets a
+			// 2-node test exercise cross-pod delivery without lining up a boresight.
+			// Off by default; the interactive beam path needs no such hook.
 			if (getenv("BT_MP_FORCE_DMG"))
 			{
 				static float s_fd = 0.0f; s_fd += dt;
@@ -2440,9 +2437,10 @@ void
 							Entity::TakeDamageMessageID,
 							sizeof(Entity::TakeDamageMessage),
 							GetEntityID(), -1, dmg);
-						DEBUG_STREAM << "[mp-force] " << (void*)m
-							<< " Dispatch unaimed TakeDamage id=" << m->GetEntityID()
-							<< " ownerID=" << (int)m->GetOwnerID() << "\n" << std::flush;
+						if (getenv("BT_MP_LOG"))
+							DEBUG_STREAM << "[mp-force] " << (void*)m
+								<< " cross-pod TakeDamage id=" << m->GetEntityID()
+								<< " -> ownerID=" << (int)m->GetOwnerID() << "\n" << std::flush;
 						m->Dispatch(&td);
 						break;
 					}