BT_EXPIRE: tester builds stop working 14 days after build (compile gate)

Distributed test zips go stale and stray.  BT_EXPIRE (CMake option,
DEFAULT ON so a zip can never accidentally ship without it) makes the
exe refuse to start BT_EXPIRE_DAYS (default 14) after its build day:
boot logs the remaining window, an expired build logs + shows a
"test build expired -- ask the operator for the current build" box and
exits.  btversion.h now stamps BT_BUILD_UNIX (UTC-midnight-rounded so
the only-on-change header churns once per day, not every build) +
BT_BUILD_DATE.  Dev is unaffected: every build re-stamps the day.

-DBT_EXPIRE=OFF builds a non-expiring exe; mkdist reads the cache and
marks such a zip "-noexpire" + prints a warning, and the tester README
expiry note is {EXPIRE}-marker-gated like the steam lines.  Second
compile-gate exception after BT_STEAM (convention updated in
glass-cockpit.md).  Quality gate, not DRM -- a clock rollback defeats it.

Verified live: default build logs "test build window: 14 day(s) left";
a -DBT_EXPIRE_DAYS=0 build logs TEST BUILD EXPIRED and parks on the
explanation box (window title checked) without starting the game;
restored to 14 and rebuilt clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

game/btl4main.cpp

@@ -24,6 +24,7 @@
 #include <iostream>
 #include <cstdlib>
 #include <cstring>
+#include <ctime>                   // BT_EXPIRE tripwire (time)
 
 #include <string>
 #include <vector>
@@ -542,6 +543,39 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
 	std::cout << "BattleTech " << BT_VERSION_FULL
 		<< " (win32 reconstruction of Tesla 4.10)" << std::endl << std::flush;
 
+#ifdef BT_EXPIRE
+	// TESTER-BUILD EXPIRY TRIPWIRE (compile gate BT_EXPIRE, default ON --
+	// see CMakeLists).  Distributed zips go stale and stray; refuse to run
+	// BT_EXPIRE_DAYS after the build day (btversion.h BT_BUILD_UNIX, UTC
+	// midnight).  Dev builds re-stamp every build, so this never fires
+	// locally.  Quality gate, not DRM.
+	{
+		const time_t expires = (time_t)BT_BUILD_UNIX
+			+ (time_t)BT_EXPIRE_DAYS * 86400;
+		const time_t now = time(0);
+		if (now >= expires)
+		{
+			std::cout << "[boot] TEST BUILD EXPIRED (built " << BT_BUILD_DATE
+				<< ", " << BT_EXPIRE_DAYS << "-day test window)"
+				<< std::endl << std::flush;
+			char expired_text[256];
+			sprintf(expired_text,
+				"This BattleTech test build (%s, built %s) has expired.\n\n"
+				"Test builds stop working %d days after they are made so\n"
+				"stale copies don't wander around.\n\n"
+				"Ask the operator for the current build.",
+				BT_VERSION_STRING, BT_BUILD_DATE, (int)BT_EXPIRE_DAYS);
+			MessageBoxA(NULL, expired_text,
+				"BattleTech -- test build expired", MB_OK | MB_ICONWARNING);
+			return 0;
+		}
+		int days_left = (int)((expires - now + 86399) / 86400);
+		std::cout << "[boot] test build window: " << days_left
+			<< " day(s) left (built " << BT_BUILD_DATE << ")"
+			<< std::endl << std::flush;
+	}
+#endif
+
 	// CPU pin (timing stability).  BT_AFFINITY overrides the mask; =0 disables --
 	// required for multi-instance runs (two instances pinned to core 0 starve
 	// each other, and the L4NET connect-retry loop busy-waits).