#149: [segperf] telemetry DEFAULT ON for every player (BT_PERF_LOG=0 opts out)

Operator call: an opt-in flag on the one machine we most need data from is
the silent-diagnostic mistake this project's doctrine exists to prevent.
Every session log now carries [segperf] beside every [rstat] window, so the
next playtest gives the 817->857 stall hunt cross-machine baselines for free
-- Oracle's stalling rig and Sauron's clean one, same night, same build.
Cost: ~2 QPC reads per segment query, microseconds per second.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018SgmXGNMXavXiafKXf9MDC

engine/MUNGA/JMOVER.cpp

@@ -135,7 +135,10 @@ void
 	)
 {
 	Check(this);
-	static const int sPerf = getenv("BT_PERF_LOG") ? 1 : 0;
+	// DEFAULT ON (operator, 2026-08-09): every player's session log should
+	// carry this -- cross-machine comparison is the point.  BT_PERF_LOG=0
+	// opts out.  Cost: ~2 QPC reads per call, microseconds per second.
+	static const int sPerf = !(getenv("BT_PERF_LOG") && *getenv("BT_PERF_LOG") == '0');
 	LARGE_INTEGER t0, t1, fq;
 	++gBTSegWCalls;
 	if (sPerf) QueryPerformanceCounter(&t0);

engine/MUNGA_L4/L4VIDEO.cpp

@@ -9039,7 +9039,7 @@ void DPLRenderer::ExecuteImplementation(RendererComplexity, RendererOrigin::Inte
 			// invalidation passes (the expensive arm the #141 sweep may have
 			// multiplied); ms = time inside the accessor for the whole window.
 			{
-				static const int sSegPerf = getenv("BT_PERF_LOG") ? 1 : 0;
+				static const int sSegPerf = !(getenv("BT_PERF_LOG") && *getenv("BT_PERF_LOG") == '0');
 				extern int gBTSegWCalls, gBTSegWDirty;
 				extern double gBTSegWMs;
 				if (sSegPerf)