Buried wreck goes INERT: collision off + target lock dropped

Answers "it's still there but invisible": not normal -- the real game removes
the dead entity (death row).  Full entity teardown needs the mech render tree
unhooked from the renderer first (the remaining P5 follow-through), so until
then the burial transition makes the wreck behaviorally gone:
  - collisionVolumeCount = 0 (the collision gather skips volume-less movers;
    MoveCollisionVolume early-outs) -> no phantom blocking
  - the player's target lock drops -> beams stop converging on / hitting the
    empty spot, no phantom impact smoke
Verified: zero [damage] hits after burial; INERT fires one-shot at
"wreck buried".

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

game/reconstructed/mech4.cpp

@@ -961,6 +961,24 @@ void
 		// authored 10s window; once buried, the smoke stops with it.
 		extern int BTWreckSinkTick(Entity *victim, float dt);
 		int wreck_visible = BTWreckSinkTick(this, (float)dt);
+
+		// BURIAL TRANSITION (one-shot): the authentic game REMOVES the dead
+		// entity (the death row) once its wreck is gone; full entity teardown is
+		// the remaining P5 follow-through (the render tree must be unhooked
+		// first), so until then the buried wreck goes INERT instead: no
+		// collision volume (stops phantom blocking -- the assistant's gather
+		// skips a mover with collisionVolumeCount 0, and MoveCollisionVolume
+		// early-outs) and no target lock (stops phantom hits/impact smoke on
+		// the empty spot).
+		if (!wreck_visible && collisionVolumeCount != 0)
+		{
+			collisionVolumeCount = 0;
+			if ((Entity *)this == gEnemyMech)
+				gEnemyMech = 0;
+			if (getenv("BT_DEATH_LOG"))
+				DEBUG_STREAM << "[death] buried wreck went INERT (collision off, "
+					"target lock dropped)\n" << std::flush;
+		}
 		wreckSmokeTimer -= dt;
 		if (wreck_visible && wreckSmokeTimer <= 0.0f)
 		{