Build: the BT_GLASS/BT_STEAM compile gates -- glass-cockpit dev-layer scaffolding

Step 1 of the desktop developer layer: option(BT_GLASS OFF) + option(BT_STEAM OFF)
(BT_STEAM requires BT_GLASS, configure-time FATAL_ERROR), defines flow to all three
targets via BT_DEFS. Defaults build = the pure pod game, verified: 3-config configure
matrix + negative test + defaults Debug build (known mech3 /FORCE externals only).
New topic context/glass-cockpit.md + ledger docs/GLASS_COCKPIT.md; router rows added.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -1,5 +1,7 @@
 # build output
 /build/
+/build-pod/
+/build-steam/
 *.obj
 *.pdb
 *.ilk
@@ -15,3 +17,8 @@ cdb_*.txt
 *.user
 Thumbs.db
 __pycache__/
+
+# glass-cockpit dev layer: per-developer / per-machine runtime files
+content/bindings.txt
+content/frontend.egg
+content/steam_appid.txt