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/InstalledPreparedCollisionCatalogTests.cs

@@ -0,0 +1,58 @@
+using AcDream.Content.Pak;
+using DatReaderWriter;
+using DatReaderWriter.Options;
+
+namespace AcDream.Content.Tests;
+
+public sealed class InstalledPreparedCollisionCatalogTests
+{
+    [Fact]
+    public void InstalledPackage_ContainsAndReadsCanonicalCollisionKeys()
+    {
+        string? datDir = ResolveDatDir();
+        if (datDir is null)
+            return;
+
+        string packagePath = Path.Combine(datDir, "acdream.pak");
+        if (!File.Exists(packagePath))
+            return;
+
+        using var dats = new DatCollection(datDir, DatAccessType.Read);
+        using var adapter = new DatCollectionAdapter(dats);
+        using var source =
+            new PakPreparedAssetSource(packagePath, adapter);
+
+        Assert.Equal(
+            PreparedAssetReadStatus.Loaded,
+            source.ReadGfxObjCollision(0x0100_0A2Bu).Status);
+        Assert.Equal(
+            PreparedAssetReadStatus.Loaded,
+            source.ReadSetupCollision(0x0200_0001u).Status);
+        Assert.Equal(
+            PreparedAssetReadStatus.Loaded,
+            source.ReadCellStructureCollision(0xA9B4_013Fu).Status);
+        Assert.Equal(
+            PreparedAssetReadStatus.Loaded,
+            source.ReadEnvCellTopology(0xA9B4_013Fu).Status);
+        Assert.Equal(4, source.CollisionStats.Loaded);
+        Assert.True(source.MappedVirtualBytes > 1L << 30);
+    }
+
+    private static string? ResolveDatDir()
+    {
+        string? configured =
+            Environment.GetEnvironmentVariable("ACDREAM_DAT_DIR");
+        if (!string.IsNullOrWhiteSpace(configured)
+            && Directory.Exists(configured))
+        {
+            return configured;
+        }
+
+        string fallback = Path.Combine(
+            Environment.GetFolderPath(
+                Environment.SpecialFolder.UserProfile),
+            "Documents",
+            "Asheron's Call");
+        return Directory.Exists(fallback) ? fallback : null;
+    }
+}