fix(streaming): preserve portal destination ownership

Detach old-world spatial ownership atomically, prioritize destination retirement dependencies, and reveal the viewport at the retail transition edge. Give private paperdoll views independent mesh ownership and retain dormant ACE entities so portal revisits preserve server objects without extending active GPU lifetimes.

src/AcDream.App/Streaming/GpuLandblockRetirement.cs

@@ -13,6 +13,17 @@ public sealed record GpuLandblockRetirement(
     LandblockRetirementKind Kind,
     IReadOnlyList<WorldEntity> Entities);
 
+/// <summary>
+/// Exact result of the atomic spatial-generation swap used before a shared
+/// world-origin recenter. Spatial membership is already unreachable when this
+/// value returns; the retained per-landblock receipts let presentation owners
+/// retire their resources later under the ordinary frame budget.
+/// </summary>
+internal sealed record GpuWorldRecenterRetirement(
+    IReadOnlyList<GpuLandblockRetirement> Landblocks,
+    int SpatialOperationCount,
+    Exception? ObserverFailure);
+
 public enum LandblockRetirementKind
 {
     Full,