Glass panel: double-buffer PaintPanel + suppress WM_ERASEBKGND (issue #13 partial)

The panel repainted control-by-control straight onto the window DC on a
100ms timer.  Compose to a memory bitmap + single BitBlt; erase suppressed.
NOT the whole fix: the reporter's flicker is monitor-dependent (clean on a
display without the game window) = D3D-vs-GDI presentation contention, plus
a 100ms-timer-vs-125ms-lamp-flash sampling beat -- directions filed on #13.

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

engine/MUNGA_L4/L4PADPANEL.cpp

@@ -243,11 +243,21 @@ static void
 	PaintPanel(HWND window)
 {
 	PAINTSTRUCT paint;
-	HDC dc = BeginPaint(window, &paint);
+	HDC winDC = BeginPaint(window, &paint);
 	unsigned long tick = GetTickCount();
 
 	RECT client;
 	GetClientRect(window, &client);
+
+	// Issue #13 (flicker): the 100ms repaint timer redrew the panel
+	// control-by-control straight onto the window DC -- visible progressive
+	// flicker at 10 Hz.  Compose the whole frame into a memory bitmap and
+	// blit it once (plus WM_ERASEBKGND suppressed in the wndproc).  The
+	// drawing body below is untouched: `dc` is now the memory DC.
+	HDC dc = CreateCompatibleDC(winDC);
+	HBITMAP backing = CreateCompatibleBitmap(winDC,
+		client.right - client.left, client.bottom - client.top);
+	HBITMAP oldBacking = (HBITMAP)SelectObject(dc, backing);
 	HBRUSH background = CreateSolidBrush(RGB(28, 28, 28));
 	FillRect(dc, &client, background);
 	DeleteObject(background);
@@ -381,6 +391,13 @@ static void
 		}
 	}
 
+	// single blit of the composed frame (issue #13)
+	BitBlt(winDC, 0, 0, client.right - client.left, client.bottom - client.top,
+		dc, 0, 0, SRCCOPY);
+	SelectObject(dc, oldBacking);
+	DeleteObject(backing);
+	DeleteDC(dc);
+
 	EndPaint(window, &paint);
 }
 
@@ -395,6 +412,9 @@ static LRESULT CALLBACK
 		PaintPanel(window);
 		return 0;
 
+	case WM_ERASEBKGND:
+		return 1;	// issue #13: PaintPanel fills the frame itself (double-buffered)
+
 	case WM_TIMER:
 		if (wparam == RepaintTimerId)
 		{