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>

tools/btversion.cmake

@@ -7,6 +7,13 @@
 #
 # Invoked: cmake -DOUT=<header> -DSRC=<repo root> -P btversion.cmake
 
+# Build DAY stamp (UTC midnight epoch) for the tester-build expiry tripwire
+# (BT_EXPIRE).  Day precision on purpose: the header is written only-on-change,
+# so a per-second stamp would churn btl4main.obj on every no-op build.
+string(TIMESTAMP BT_NOW_UNIX "%s" UTC)
+math(EXPR BT_BUILD_UNIX "(${BT_NOW_UNIX} / 86400) * 86400")
+string(TIMESTAMP BT_BUILD_DATE "%Y-%m-%d" UTC)
+
 execute_process(COMMAND git rev-list --count HEAD
                 WORKING_DIRECTORY "${SRC}"
                 OUTPUT_VARIABLE BT_BUILD OUTPUT_STRIP_TRAILING_WHITESPACE
@@ -44,6 +51,8 @@ set(CONTENT "// generated by tools/btversion.cmake -- do not edit, not in git
 #define BT_VERSION_STRING  \"4.11.${BT_BUILD}\"
 #define BT_VERSION_HASH    \"${BT_HASH}${BT_DIRTY}\"
 #define BT_VERSION_FULL    \"4.11.${BT_BUILD} (${BT_HASH}${BT_DIRTY})\"
+#define BT_BUILD_UNIX      ${BT_BUILD_UNIX}
+#define BT_BUILD_DATE      \"${BT_BUILD_DATE}\"
 ")
 
 # write only when changed so incremental builds don't churn

tools/mkdist.py

@@ -54,6 +54,7 @@ def main():
     # the CMake cache; a steam-OFF zip must carry neither play_steam.bat nor
     # steam_api.dll (a stale DLL can linger from an earlier ON build).
     steam_on = False
+    cache = ""
     try:
         cache = open("build/CMakeCache.txt", encoding="utf-8", errors="ignore").read()
         steam_on = "BT_STEAM:BOOL=ON" in cache
@@ -62,6 +63,15 @@ def main():
     print("  BT_STEAM=%s (from build/CMakeCache.txt)" % ("ON" if steam_on else "OFF"))
     if not steam_on:
         name += "-nosteam"      # distinct name: never clobbers the dev-flavor zip
+
+    # TESTER-BUILD EXPIRY (BT_EXPIRE, default ON): a zip cut from an
+    # expire-OFF build never dies in the field -- mark it loudly so it is a
+    # deliberate act, not an accident.
+    expire_on = "BT_EXPIRE:BOOL=ON" in cache if cache else True
+    print("  BT_EXPIRE=%s (from build/CMakeCache.txt)" % ("ON" if expire_on else "OFF"))
+    if not expire_on:
+        name += "-noexpire"
+        print("  WARNING: this zip NEVER EXPIRES -- intended for distribution?")
     zpath = os.path.join(outdir, name + ".zip")
 
     exes = ["build/Release/btl4.exe"]
@@ -102,6 +112,10 @@ def main():
                 lines = [l.replace("{STEAM}", "") for l in lines]
             else:                             # steam-free zip: strip them
                 lines = [l for l in lines if "{STEAM}" not in l]
+            if expire_on:                     # same treatment for expiry note
+                lines = [l.replace("{EXPIRE}", "") for l in lines]
+            else:
+                lines = [l for l in lines if "{EXPIRE}" not in l]
             readme = "".join(lines)
             z.writestr(name + "/README.txt", readme)
         for p in bats:                       # bats at the zip ROOT (next to