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/L4GREND.cpp

@@ -7,7 +7,9 @@
 #include "l4gauge.h"
 #include "..\munga\mission.h"
 #include "l4plasma.h"
+#ifdef BT412
 #include "l4plasmascreen.h"
+#endif
 #include "..\munga\notation.h"
 
 // #define LOCAL_TEST
@@ -103,8 +105,15 @@ L4GaugeRenderer::L4GaugeRenderer(bool windowed, int *secondaryIndex, int *aux1In
 			//}
 			if (_stricmp(plasma_string, "SCREEN") == 0)
 			{
+#ifdef BT412
 				Tell("Plasma display created as an on-screen window\n");
 				externalDisplay = new PlasmaScreen();
+#else
+				// BT412=OFF (pod-minimal build): the desktop PlasmaScreen is
+				// compiled out; no external display (Register below is
+				// NULL-guarded), as on BT411.
+				Tell("L4PLASMA=SCREEN ignored: PlasmaScreen not in this build (BT412=OFF)\n");
+#endif
 			}
 			else
 			{