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>

tests/AcDream.App.Tests/Rendering/RenderFrameOrchestratorTests.cs

@@ -59,6 +59,31 @@ public sealed class RenderFrameOrchestratorTests
         Assert.Equal(outcome, phases.ObservedOutcome);
     }
 
+    [Theory]
+    [InlineData(0, 1080)]
+    [InlineData(1920, 0)]
+    [InlineData(0, 0)]
+    [InlineData(-1, 1080)]
+    public void ZeroAreaViewport_SkipsTheFrameBeforeAnyGpuWork(int width, int height)
+    {
+        // Campaign VM VM7 owner gate (2026-08-23): alt-tabbing out of
+        // exclusive fullscreen auto-iconifies the window; for one frame the
+        // size reads 0x0 while the swapchain is still current, and the
+        // zero-area frame crashed the client in
+        // RenderPackActivationExtent.Validate. The rule lives here, before
+        // BeginFrame, so no phase, measurement, diagnostics or recovery can
+        // observe a zero extent, and the outcome says so for the host.
+        var calls = new List<string>();
+        var phases = new RecordingPhases(calls);
+
+        RenderFrameOutcome outcome = Create(phases).Render(
+            Input with { ViewportWidth = width, ViewportHeight = height });
+
+        Assert.True(outcome.SkippedZeroArea);
+        Assert.Empty(calls);
+        Assert.Empty(phases.ObservedInputs);
+    }
+
     [Fact]
     public void BeginFailure_DoesNotAttemptAnyPhaseOrClose()
     {