#84: the replicant salvo now HOMES AT THE LIVE TARGET -- the stale-point double detonation is gone

Oracle (674): "missile appear to register hit explosions twice, once where
target was and again where the target is".  Rajel (693): "once where the mech
was when I fired and on the expected impact".

MECHANISM, measured on two nodes (BT_PROJ_LOG "DET at" now logs EVERY contact
detonation, including damage-0 rounds, which were invisible before): the
replicant salvo mirror pushed rounds with NO entity handle at the FROZEN
fire-time aim point, and the pool's contact test is proximity to p.targetPos --
so mirror rounds "contacted" the empty air where the target USED to be and
detonated there, on every peer, every salvo (45 salvos -> 234 frozen-point
air bursts in a 200s bench).  The true impact appears at the live position
(the shooter's own homing rounds on his node; the victim's damage reaction on
peers) -- two sites per salvo, exactly as reported.

THE BINARY'S MODEL (CLASSMAP, Missile flight-entity cluster): missiles are
ENTITIES with authoritative/GHOST Performance variants (Projectile
PTR_LAB_005129e8 / 005129f4) and their own WriteUpdateRecord (@4bef4c, slot 7)
-- peers ran ghost missiles updated from the wire and saw the TRUE trajectory.
One explosion, correct place, every node.  The frozen-point mirror is the port
infidelity (the entity Missile itself stays blocked by the documented 2007
Entity-base mismatch; mislanch.cpp:301).

FIX: the MissileLauncher update record now carries the locked target's EntityID
(salvoTargetID; EntityID::Null = point fire).  The mirror resolves it on the
receiving node via HostManager::GetEntityPointer (engine T0, an index-socket
Find -- NULL-safe on any ID), refuses non-mechs via BTIsRegisteredMech, and
pushes the mirror rounds WITH the handle -- they re-lead on the live local
replicant exactly as the master's own rounds do, and detonate at the true
position.  Guards: record-length gate (a short record from an older build has
no ID field -- its tail would be garbage and must not resolve) + Null check +
registered-mech check.

VERIFIED (two nodes, 200s, missile autofire both ways):
  * every record arrives len=52/52 with sane aims ([mlrec] diag)
  * 22/46 salvos resolve (tgtID=3:22 -> the victim's local master) and their
    mirror rounds home: the frozen-air DET population fell 234 -> 102, the
    live-homing population rose to 155/207 per node
  * the unresolved remainder is BENIGN and correct: EntityID::Null point fire
    plus hostID=-1 LOCAL entities -- i.e. the victim's WRECK, which autofire
    keeps shooting after the kill; a wreck does not move, so the frozen aim IS
    its true position
Downstream expectations: Ronin's smoke-screen (#114) should drop by ~the mirror
half, and the audio census (#32) loses the duplicate Static3DPatchSource
explosion sounds on peers -- both to be read from the next field logs.

Also in this commit: [projectile] DET diagnostic (every contact detonation with
position/damage/handle/aim), [mlrec] record-arrival diagnostic (len/recID/
subsys/counter/aim, capped 200), scratchpad/night9/mp_double.sh.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

game/reconstructed/mech4.cpp

@@ -1676,6 +1676,16 @@ static void
 		}
 		if (contact)
 		{
+			// #84 diag: EVERY contact detonation, incl. damage-0 visual/mirror
+			// rounds (the IMPACT line below only covers damage-carrying rounds,
+			// which hid the replicant mirror's stale-point bursts entirely).
+			if (getenv("BT_PROJ_LOG"))
+				DEBUG_STREAM << "[projectile] DET at(" << hitPos.x << ","
+					<< hitPos.y << "," << hitPos.z << ")"
+					<< " dmg=" << p.damage
+					<< " tgt=" << (void *)p.target
+					<< " aim(" << p.targetPos.x << "," << p.targetPos.y
+					<< "," << p.targetPos.z << ")" << std::endl;
 			BTSpawnRoundDetonation(p.shooter, p.weaponSubsys, hitPos);
 			Entity *tgt = p.target;
 			// Deliver to the projectile's target mech -- the launcher set p.target