feat(content): bake and read flat collision assets

Append strict collision/topology payloads to the existing prepared package so later physics cutover can drop parsed DAT graphs without adding a second mapping or changing traversal behavior. The full 2,232,170-key catalog is deterministic across worker counts, exact-byte aliased, corruption-isolated, and cancellation-safe.

tests/AcDream.Content.Tests/PakKeyTests.cs

@@ -35,6 +35,10 @@ public class PakKeyTests {
     [InlineData(PakAssetType.GfxObjMesh, 1)]
     [InlineData(PakAssetType.SetupMesh, 2)]
     [InlineData(PakAssetType.EnvCellMesh, 3)]
+    [InlineData(PakAssetType.GfxObjCollision, 4)]
+    [InlineData(PakAssetType.SetupCollision, 5)]
+    [InlineData(PakAssetType.CellStructureCollision, 6)]
+    [InlineData(PakAssetType.EnvCellTopology, 7)]
     public void AssetType_NumericValuesAreStable(PakAssetType type, byte expected) {
         // These values are a wire format — pin them so a future refactor can't
         // silently renumber the enum and corrupt existing paks.