feat(streaming): shadow-publish flat collision assets

Carry one immutable prepared collision closure with each accepted near-tier generation and install graph plus flat views through the same retained publication receipt. Apply the same strict package-only rule to live entities, add exact sampled graph-authoritative comparison artifacts and lifecycle counters, and prove cancellation, demotion, rehydrate, revisit, teardown, reconnect, and the nine-stop route with 14,064 zero-mismatch samples.

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

src/AcDream.Core/World/LandblockCollisionBuild.cs

@@ -0,0 +1,29 @@
+using System.Collections.Immutable;
+using AcDream.Core.Physics;
+
+namespace AcDream.Core.World;
+
+/// <summary>
+/// Immutable prepared-collision closure carried by one near-tier landblock
+/// build. Keys remain the original DAT source IDs. Cell-structure geometry and
+/// per-EnvCell topology stay separate so package aliases retain one reusable
+/// geometry identity without conflating cell placement or portal state.
+/// </summary>
+public sealed record LandblockCollisionBuild(
+    ImmutableDictionary<uint, FlatGfxObjCollisionAsset> GfxObjs,
+    ImmutableDictionary<uint, FlatSetupCollision> Setups,
+    ImmutableDictionary<uint, FlatCellStructureCollisionAsset> CellStructures,
+    ImmutableDictionary<uint, FlatEnvCellTopology> EnvCells,
+    ImmutableArray<uint> GfxObjIds,
+    ImmutableArray<uint> SetupIds,
+    ImmutableArray<uint> EnvCellIds)
+{
+    public static readonly LandblockCollisionBuild Empty = new(
+        ImmutableDictionary<uint, FlatGfxObjCollisionAsset>.Empty,
+        ImmutableDictionary<uint, FlatSetupCollision>.Empty,
+        ImmutableDictionary<uint, FlatCellStructureCollisionAsset>.Empty,
+        ImmutableDictionary<uint, FlatEnvCellTopology>.Empty,
+        ImmutableArray<uint>.Empty,
+        ImmutableArray<uint>.Empty,
+        ImmutableArray<uint>.Empty);
+}