fix #424: a zero-area viewport skips the frame before any GPU work (alt-tab out of exclusive fullscreen crashed the pack-on client)

GLFW auto-iconifies an exclusive-fullscreen window on focus loss; for one
frame the window size reads 0x0 while the swapchain is still created, so
GameWindow.OnRender's PrepareFrame guard let a zero-area RenderFrameInput
through. The retail path tolerated it silently; the render-pack controller
correctly rejects a zero activation extent, which surfaced the latent frame
as an unhandled ArgumentOutOfRangeException during the owner's VM6/VM3 gate.

RenderFrameOrchestrator.Render now returns RenderFrameOutcome.ZeroArea
before BeginFrame when either dimension is <= 0 (no GPU frame, phase,
measurement, diagnostics or recovery runs) and GameWindow skips
NoteFrameClosed for it. Test: ZeroAreaViewport_SkipsTheFrameBeforeAnyGpuWork.

Verify: App hermetic lane 6,059/0 (Release).

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

src/AcDream.App/Rendering/RenderFrameOrchestrator.cs

@@ -27,9 +27,23 @@ internal readonly record struct PrivatePresentationFrameOutcome(
     bool ScreenshotCaptured);
 
 /// <summary>The immutable result observed by post-screenshot diagnostics.</summary>
+/// <param name="SkippedZeroArea">
+/// True when the frame was never opened because the viewport had no area
+/// (Campaign VM VM7 owner gate, 2026-08-23: alt-tabbing out of exclusive
+/// fullscreen auto-iconifies the GLFW window, so for one frame the window
+/// size reads 0x0 while the swapchain is still "created" and the host's
+/// minimised guard has not yet armed; the zero-area frame then reached
+/// <c>RenderPackActivationExtent.Validate</c> and crashed the client). No
+/// GPU frame, measurement, phase, diagnostics or recovery ran for such a
+/// frame, so the host must not note it as closed.
+/// </param>
 internal readonly record struct RenderFrameOutcome(
     WorldRenderFrameOutcome World,
-    PrivatePresentationFrameOutcome Presentation);
+    PrivatePresentationFrameOutcome Presentation,
+    bool SkippedZeroArea = false)
+{
+    internal static RenderFrameOutcome ZeroArea => new(default, default, SkippedZeroArea: true);
+}
 
 internal interface IRenderFrameLifetime
 {
@@ -191,6 +205,13 @@ internal sealed class RenderFrameOrchestrator : IGameRenderFrameRoot
 
     public RenderFrameOutcome Render(RenderFrameInput input)
     {
+        // A window with no area has nothing to render and no swapchain to
+        // render into; decide that BEFORE a GPU frame is opened so nothing
+        // downstream (the render-pack activation extent, the world target,
+        // the presentation viewport) ever sees a zero extent.
+        if (input.ViewportWidth <= 0 || input.ViewportHeight <= 0)
+            return RenderFrameOutcome.ZeroArea;
+
         _lifetime.BeginFrame();
         Exception? renderFailure = null;
         try