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>

run/limpdemo.cmd

@@ -0,0 +1,20 @@
+@echo off
+rem #78 OBS capture demo: Black Hawk walks ~12 s, right leg crosses half on
+rem the FINAL damage tick (one clean trigger edge), then: klaxon, klaxon,
+rem "REVERSE DISABLED" + the limp.  Auto-walks via GOTO -- the only input
+rem needed is V (or BACKTICK) once, for the 3rd-person chase view.
+setlocal
+set BT_FE_SOLO=1
+set BT_GOTO=8000 8000
+set BT_GOTO_THR=0.9
+set BT_SELF_DAMAGE=6
+set BT_SELF_DAMAGE_ZONE=18
+set BT_SELF_DAMAGE_TICKS=12
+set BT_SELF_DAMAGE_DELAY=30
+set BT_KEY_NOFOCUS=1
+cd /d "%~dp0..\content"
+powershell -NoProfile -Command "(Get-Content DEV.EGG) -replace '^experience=.*','experience=novice' | Set-Content LIMPDEMO.EGG"
+echo [limpdemo] auto-walk, 6x12 into zone 18 (crossing on the last tick), novice
+"%~dp0..\build\Release\btl4.exe" -egg LIMPDEMO.EGG
+del LIMPDEMO.EGG 2>nul
+if errorlevel 1 pause