demo/bench harness polish: BT_SELF_DAMAGE_DELAY + two harness bug fixes + the OBS limp demo bat

- BT_SELF_DAMAGE_DELAY=<s>: hold the harness off for n seconds (capture demos:
  healthy walking first).  Fixed its underflow bug (the countdown crossed below
  zero, which is also the "read the env" sentinel -- the delay re-armed forever).
- The one-death latch was fed by EVERY mech's update: a mission that generated
  any destroyed mech entity tripped it at frame one and the harness silently
  never fired (random per mission roll).  Now viewpoint-mech only.
- run\limpdemo.cmd: the OBS capture demo -- 30 s healthy auto-walk, then the
  leg crosses half on the final damage tick: klaxon-klaxon-"reverse disabled"
  + the limp, then unlimited limp footage (waypoint 8000,8000).

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

game/reconstructed/mech4.cpp

@@ -5627,7 +5627,12 @@ void
 		// shooting us.  (Live damage knocks the power bus down, which is exactly
 		// the signal a respawn test is trying to read.)
 		static int s_sdSpent = 0;
-		if (IsMechDestroyed())
+		// Latch off at the VIEWPOINT mech's first death only.  (The latch used
+		// to be fed by EVERY mech's update -- a mission that generated any
+		// destroyed mech/wreck entity tripped it at frame one and the harness
+		// silently never fired; random per mission roll.)
+		if ((Entity *)this == application->GetViewpointEntity()
+			&& IsMechDestroyed())
 			s_sdSpent = 1;
 		if ((Entity *)this == application->GetViewpointEntity()
 			&& getenv("BT_SELF_DAMAGE") && !s_sdSpent
@@ -5635,6 +5640,20 @@ void
 		{
 			static float s_sdAccum = 0.0f;
 			static int   s_sdTicks = 0;
+			// BT_SELF_DAMAGE_DELAY=<s>: hold the harness off for the first n
+			// seconds of the mission (capture/demo: healthy walking first).
+			static float s_sdDelay = -1.0f;
+			if (s_sdDelay < 0.0f)
+			{
+				const char *de = getenv("BT_SELF_DAMAGE_DELAY");
+				s_sdDelay = (de != 0 && *de != '\0') ? (float)atof(de) : 0.0f;
+			}
+			if (s_sdDelay > 0.0f)
+			{
+				s_sdDelay -= dt;
+				if (s_sdDelay < 0.0f) s_sdDelay = 0.0f;	// NOT below: <0 is the
+				s_sdAccum = 0.0f;						// "read the env" sentinel --
+			}											// underflow re-armed the delay forever
 			// BT_SELF_DAMAGE_TICKS=<n>: stop after n hits (bench: ramp a leg
 			// PAST LegHalfStructure then hold, so the limp window is unbounded
 			// instead of the 1 s between half and destroyed at high dps).