BT_STEAM restored as THE compile gate (license) -- default OFF

The unification flattened BT_STEAM into the always-on exe; the user flagged
the constraint that makes it the one legitimate compile gate: the Steamworks
SDK license bars a public release with Steam inside, so a release build must
exclude it entirely.

- CMakeLists: option(BT_STEAM OFF) restored around the Steamworks block --
  an OFF configure touches no SDK headers, no steam_api.lib, no DLL copy.
  BT_GLASS stays retired (always defined); ONE build dir either way.
  Dev checkout: build/ configured once with -DBT_STEAM=ON.
- mkdist: reads BT_STEAM from build/CMakeCache.txt.  Steam-OFF zips exclude
  play_steam.bat + steam_api.dll (explicitly filtering a STALE DLL left by
  an earlier ON build), strip the README steam section ({STEAM} line
  markers), and take a -nosteam name suffix so the flavors never clobber
  (found live: both stamped 4.11.436 and the release zip overwrote the dev
  zip).

Verified both flavors:
  ON  (dev build):  steam smoke degrades to Winsock without the client;
      zip carries play_steam.bat + steam_api.dll + README steam lines.
  OFF (scratch build-relcheck, deleted after): 0 compile errors; exe dir =
      btl4.exe + OpenAL32.dll only; boots + simulates; BT_STEAM_NET=1 inert
      (transport not in the binary); end-to-end zip has zero steam bits and
      a clean README.

KB: context/glass-cockpit.md unified-build section records the license gate.

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

tools/mkdist.py

@@ -43,17 +43,31 @@ def main():
         githash = subprocess.check_output(
             ["git", "rev-parse", "--short", "HEAD"]).decode().strip()
         name = "BT411-%s-%s" % (time.strftime("%Y-%m-%d"), githash)
-    zpath = os.path.join(outdir, name + ".zip")
 
     tracked = subprocess.check_output(
         ["git", "ls-files", "-z", "content/"]).decode().split("\0")
     tracked = [t for t in tracked if t]
 
-    # ONE unified build (2026-07-21): every layer (pod game + glass cockpit +
-    # Steam transport) lives in the single exe; steam_api.dll is delay-loaded.
+    # ONE unified build (2026-07-21): every layer lives in the single exe.
+    # THE LICENSE GATE: BT_STEAM is compile-time (Steamworks SDK terms bar a
+    # public release with Steam inside).  Read the configure's answer from
+    # 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
+    try:
+        cache = open("build/CMakeCache.txt", encoding="utf-8", errors="ignore").read()
+        steam_on = "BT_STEAM:BOOL=ON" in cache
+    except OSError:
+        pass
+    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
+    zpath = os.path.join(outdir, name + ".zip")
+
     exes = ["build/Release/btl4.exe"]
-    bats = ["players/play_solo.bat", "players/join.bat", "players/join_lan.bat",
-            "players/play_steam.bat"]
+    bats = ["players/play_solo.bat", "players/join.bat", "players/join_lan.bat"]
+    if steam_on:
+        bats.append("players/play_steam.bat")
     for p in exes + bats:
         if not os.path.exists(p):
             sys.exit("MISSING: %s -- build/refresh it first" % p)
@@ -63,8 +77,12 @@ def main():
     for exe in list(exes):
         d = os.path.dirname(exe)
         for f in sorted(os.listdir(d)):
-            if f.lower().endswith(".dll"):
-                exes.append(d + "/" + f)
+            if not f.lower().endswith(".dll"):
+                continue
+            if f.lower() == "steam_api.dll" and not steam_on:
+                print("  (steam OFF: leaving stale %s out of the zip)" % f)
+                continue
+            exes.append(d + "/" + f)
 
     # System runtime DLLs the exe needs on a bare tester machine (32-bit
     # d3dx9 + MSVC runtimes -- carried in redist/, shipped next to the exe;
@@ -79,6 +97,12 @@ def main():
         if os.path.exists("players/README.txt"):
             readme = open("players/README.txt", encoding="utf-8").read()
             readme = readme.replace("{VERSION}", name.replace("BT411_", ""))
+            lines = readme.splitlines(True)
+            if steam_on:                      # keep steam lines, drop marker
+                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]
+            readme = "".join(lines)
             z.writestr(name + "/README.txt", readme)
         for p in bats:                       # bats at the zip ROOT (next to
             arc = os.path.basename(p)        # content\ + build\, per the guard)