plasma window: initialize the display area (junk shown until the callsigns)

The desktop plasma marquee showed heap garbage from boot until the gauge
renderer first drew the callsigns.  Root cause [T0]: PixelMap8(w,h) allocates
its pixel bytes UNCLEARED (GRAPH2D.cpp `new Byte[w*h]`, no memset) and
Video8BitBuffered never clears them either -- only changedLine[] is zeroed.
The real pod never showed it: the serial PlasmaDisplay transfers only CHANGED
lines, so the garbage never left the buffer and the hardware marquee sat at
its power-on blank.  PlasmaWindow::Update however blits the WHOLE buffer
every frame, exposing the junk for the window-up-to-first-draw gap (seconds
on slower rigs).

Fix: PlasmaWindow ctor memsets the pixelBuffer to index 0 (the near-black
background) -- the window comes up blank like the hardware did.  Glass-layer
only; the serial pod path is untouched.  Verified: glass boot shows a clean
marquee from the first visible frame.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/MUNGA_L4/L4PLASMAWIN.cpp

@@ -54,6 +54,22 @@ PlasmaWindow::PlasmaWindow():
 	}
 	blitBuffer = new unsigned long[plasmaWidth * plasmaHeight];
 	memset(blitBuffer, 0, plasmaWidth * plasmaHeight * sizeof(unsigned long));
+
+	//
+	// INITIALIZE THE DISPLAY AREA (2026-08-10).  PixelMap8(w,h) allocates its
+	// pixel bytes UNCLEARED (GRAPH2D.cpp: `new Byte[w*h]`, no memset), so the
+	// 128x32 plasma buffer starts as heap garbage.  The real pod never showed
+	// it: the serial PlasmaDisplay transfers only CHANGED lines (changedLine[]
+	// is zeroed by the base ctor), so the garbage never left the buffer and
+	// the hardware marquee sat at its power-on blank.  This desktop window
+	// blits the WHOLE buffer every frame, so the junk was visible from boot
+	// until the gauge renderer first drew the callsigns.  Clear to index 0
+	// (the near-black background) -- the window comes up blank like the
+	// hardware did.
+	//
+	if (pixelBuffer != NULL && pixelBuffer->Data.MapPointer != NULL)
+		memset(pixelBuffer->Data.MapPointer, 0, plasmaWidth * plasmaHeight);
+
 	DEBUG_STREAM << "[plasmawin] desktop plasma display up (scale x"
 		<< scale << ")\n" << std::flush;
 }