Build: the BT412 compile gate -- one tree builds the pod game and the Steam game

option(BT412 ... ON) gates the steamification layer for the merge-back into
BT411, so the unified repo serves both fronts:

- Default (BT412=ON): pod/dev game + PadRIO / plasma / front end / lobby --
  all runtime-gated, so a -egg/-net launch still behaves as 4.11.
- Pod-minimal (-DBT412=OFF, build-pod/): compiles OUT L4PADRIO, L4PADBINDINGS,
  L4KEYLIGHT, L4PLASMASCREEN, L4STEAMTRANSPORT, btl4fe, btl4console, btl4lobby.
  Seam sites carry #ifdef BT412: L4CTRL.cpp PAD token (log + ignore), L4GREND.cpp
  L4PLASMA=SCREEN (log + ignore), L4VB16.cpp (inert PadRIO stub -- call sites
  identical in both flavors), btl4main.cpp (front-end/marshal/lobby blocks out;
  zero-arg launch = 4.11 behavior).
- The NetTransport seam ALWAYS compiles -- it is the wire for both flavors
  (WinsockNetTransport, verified byte-identical to the pre-seam arcade path).
- BT412_STEAM now requires BT412 (configure-time FATAL_ERROR; negative-tested).

Verified: OFF -- clean v143 link, solo DEV.EGG mission runs (31 subsystems
tick, gait, targeting). ON -- zero-arg launch engages the front-end menu.
STEAM -- links clean, ships steam_api.dll.

Also: README/context docs for the flavors + the merge-back; corrected the stale
Phase 3b DEFERRED note (the single-window cockpit landed 07-16) and the README's
leftover 0xBD3 valve-gate claim (it is a damage/explosion hub).

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

game/btl4main.cpp

@@ -272,8 +272,10 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
 	// platform-profile block, so the DEV keyboard default (below) does not win
 	// (that left the RIO controls inert).  An explicit L4CONTROLS still
 	// overrides (e.g. L4CONTROLS=RIO for a real serial board).
+#ifdef BT412
 	if (getenv("L4MFDSPLIT") != NULL && getenv("L4CONTROLS") == NULL)
 		putenv("L4CONTROLS=PAD");
+#endif
 
 	int gBTPlatformPod = 0;
 	{
@@ -552,7 +554,12 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
 	// per mission, and it sidesteps BT's in-process re-init fragility (the
 	// reconstructed gBT* globals hold per-mission entity pointers that a
 	// second in-process mission would trip over).  `-egg`/`-net` = single pass.
+	//
+	// BT412=OFF (pod-minimal build): the front end / marshal / lobby are
+	// compiled out -- a zero-arg launch behaves as on BT411 (no menu), and
+	// `-egg`/`-net` runs are identical in both flavors.
 	//=======================================================================//
+#ifdef BT412
 	extern int  BTFrontEnd_Run();
 	extern int  BTFrontEnd_LastMissionSeconds();
 	extern int  BTFrontEnd_LastLaunchMode();
@@ -630,6 +637,7 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
 			          << secs << "s)" << std::endl << std::flush;
 		}
 	}
+#endif	// BT412
 
 	{
 		std::cout << "[boot] opening btl4.res..." << std::endl << std::flush;
@@ -677,6 +685,7 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
 
 	Stop_Registering();
 
+#ifdef BT412
 	//-------------------------------------------------------------------//
 	// LAUNCHER ROLE: if the marshal ended this front-end mission (the
 	// selected time expired), relaunch a fresh instance -- it front-end-modes
@@ -714,6 +723,7 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
 			          << std::endl << std::flush;
 		}
 	}
+#endif	// BT412
 
 	return Exit_Code;
 }