Front end: local mission-egg builder + front-end mode (Workstream B foundation)

The core 'single-player without the operator console' capability: a
zero-argument launch now builds its own mission and plays.

- game/reconstructed/btl4fe.{hpp,cpp}: the BattleTech catalog (8 maps,
  8 mechs + variants, colors) and BTFeMission_WriteEgg -- emits the exact
  console egg format (verified against content/MP.EGG): [mission],
  [ordinals] + the 4 static rank-place plasma bitmaps, [pilots],
  per-pilot loadout sections, and [largebitmap]/[smallbitmap] with
  GDI-rendered 128x32 / 64x16 pilot-name plasma bitmaps.
- L4APP.H: SetEggNotationFileName / SetNetworkCommonFlatAddress setters
  (from RP412) so the front end feeds the standard -egg load path.
- btl4main.cpp WinMain: front-end mode -- no -egg and no -net builds
  frontend.egg locally (BTFrontEnd_Run) and boots it. -egg/-net runs
  are untouched.

Verified: no-args launch -> '[frontend] built frontend.egg' -> mission
loads and runs; the generated egg is structurally identical to MP.EGG
(32x32/16x16 name bitmaps with real glyph pixels, all sections present).

Remaining in Phase 5 (deferred): the interactive on-screen catalog menu,
the in-process LocalConsole marshal (mission clock + StopMission at
expiry + single-binary loop), and the score-intake/results screen (new
for BT -- no mission-end flow exists yet). (Phase 5 of the roadmap)

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

game/btl4main.cpp

@@ -350,6 +350,30 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
 		return 1;
 	}
 
+	//
+	// FRONT-END MODE (BT412 Phase 5): no egg and no network address on the
+	// command line means there is no operator console to hand us a mission.
+	// Build one locally -- the in-game front end assembles a mission egg from
+	// the default loadout and points the standard -egg load path at it.  The
+	// interactive on-screen menu and the in-process marshal layer on top of
+	// this (docs/BT412-ROADMAP.md Phase 5).  `-egg`/`-net` runs are untouched.
+	//
+	{
+		const char *cmdline_egg = L4Application::GetEggNotationFileName();
+		const int   no_egg = (!cmdline_egg || !strlen(cmdline_egg));
+		const int   no_net = (L4Application::GetNetworkCommonFlatAddress() == 0);
+		if (no_egg && no_net)
+		{
+		extern int BTFrontEnd_Run();
+		if (BTFrontEnd_Run())
+			std::cout << "[frontend] built frontend.egg from the default loadout"
+			          << std::endl << std::flush;
+		else
+			std::cout << "[frontend] egg build FAILED -- falling through"
+			          << std::endl << std::flush;
+		}
+	}
+
 	Start_Registering();
 
 	// Create the main window (single 800x600 view for dev; pod multi-monitor is Phase 8).