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>

engine/MUNGA_L4/L4VB16.cpp

@@ -1,5 +1,17 @@
 #include "mungal4.h"
+#ifdef BT412
 #include "l4padrio.h"		// PadRIO::GetLampState / SetScreenButton (cockpit buttons)
+#else
+// BT412=OFF (pod-minimal build): PadRIO is compiled out.  This inert stub
+// keeps every call site below identical in both flavors -- the on-screen
+// cockpit buttons draw unlit and screen-button presses are ignored.
+struct PadRIO
+{
+	static int  IsActive()                 { return 0; }
+	static int  GetLampState(int)          { return 0; }
+	static void SetScreenButton(int, int)  {}
+};
+#endif
 #pragma hdrstop
 
 #include "l4vb16.h"