perf(streaming): reserve destination reveal capacity

Join destination scheduling to the canonical reveal generation, protect its share across every typed frame-budget dimension, and prevent stale work from clearing a replacement reservation. Remove forced incomplete materialization and project retail's centered portal wait cue while the authored tunnel remains active.

Tests: Release build clean; 91 focused reservation/reveal tests; full solution 8,158 passed, 5 skipped.

Co-authored-by: Codex <noreply@openai.com>

tests/AcDream.App.Tests/UI/PortalWaitNoticeControllerTests.cs

@@ -0,0 +1,37 @@
+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+public sealed class PortalWaitNoticeControllerTests
+{
+    [Fact]
+    public void Notice_IsCenteredOverlayAndDisposesFromRetainedRoot()
+    {
+        var root = new UiRoot
+        {
+            Width = 1280f,
+            Height = 720f,
+        };
+
+        using (var controller = new PortalWaitNoticeController(root))
+        {
+            UiText text = Assert.IsType<UiText>(Assert.Single(root.Children));
+            Assert.False(text.Visible);
+            Assert.True(text.Centered);
+            Assert.True(text.ClickThrough);
+            Assert.Equal(root.Width, text.Width);
+            Assert.Equal(root.Height, text.Height);
+
+            controller.Set("In Portal Space - Please Wait...");
+
+            Assert.True(text.Visible);
+            UiText.Line line = Assert.Single(text.LinesProvider!());
+            Assert.Equal("In Portal Space - Please Wait...", line.Text);
+
+            controller.Set(null);
+            Assert.False(text.Visible);
+        }
+
+        Assert.Empty(root.Children);
+    }
+}