mkdist: include the Steam build + play_steam.bat when present

Rebuilt build-steam fresh (was stale at Jul 20 -- predated today's gauge/
trigger-config/keymap fixes); the DLL scan picks up steam_api.dll
automatically.  Clean-extract verified: the steam exe boots to the front-end
menu (Steam client only needed for live lobbies).

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

tools/mkdist.py

@@ -55,6 +55,15 @@ def main():
         if not os.path.exists(p):
             sys.exit("MISSING: %s -- build/refresh it first" % p)
 
+    # Steam build (EXPERIMENTAL): included when present -- play_steam.bat
+    # hosts/joins internet sessions through the Steam lobby (needs the Steam
+    # client running; steam_api.dll ships next to the exe).
+    if os.path.exists("build-steam/Release/btl4.exe"):
+        exes.append("build-steam/Release/btl4.exe")
+        bats.append("players/play_steam.bat")
+    else:
+        print("  (no build-steam -- play_steam.bat left out)")
+
     # Runtime DLLs living next to each exe (OpenAL32.dll etc.) -- the exe
     # fails to load without them (caught by the clean-extract boot test).
     for exe in list(exes):