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.

src/AcDream.Content/Pak/PakWriter.cs

@@ -46,28 +46,51 @@ public sealed class PakWriter : IDisposable {
     /// </summary>
     public void AddBlob(ulong key, ObjectMeshData data) {
         ThrowIfFinished();
-        if (!_seenKeys.Add(key)) {
-            throw new ArgumentException($"duplicate pak key 0x{key:X16}", nameof(key));
-        }
+        ReserveKey(key);
 
-        long offset = _stream.Position;
         using var ms = new MemoryStream();
         ObjectMeshDataSerializer.Write(data, ms);
         var bytes = ms.ToArray();
+        AddReservedBlob(key, bytes);
+    }
 
+    /// <summary>
+    /// Adds an already serialized immutable payload. This is the package seam
+    /// used by typed non-render assets such as flat collision records.
+    /// </summary>
+    public void AddBlob(ulong key, ReadOnlySpan<byte> bytes) {
+        ThrowIfFinished();
+        ReserveKey(key);
+        AddReservedBlob(key, bytes);
+    }
+
+    private void AddReservedBlob(ulong key, ReadOnlySpan<byte> bytes) {
+        if ((ulong)bytes.Length > uint.MaxValue) {
+            throw new ArgumentException(
+                "one pak payload cannot exceed UInt32.MaxValue bytes",
+                nameof(bytes));
+        }
+
+        long offset = _stream.Position;
         _stream.Write(bytes);
         PadToAlignment();
 
         var receipt = new PakTocEntry {
             Key = key,
             Offset = (ulong)offset,
-            Length = (uint)bytes.Length,
+            Length = checked((uint)bytes.Length),
             Crc32 = Content.Pak.Crc32.Compute(bytes),
         };
         _tocEntries.Add(receipt);
         _receiptByKey.Add(key, receipt);
     }
 
+    private void ReserveKey(ulong key) {
+        if (!_seenKeys.Add(key)) {
+            throw new ArgumentException($"duplicate pak key 0x{key:X16}", nameof(key));
+        }
+    }
+
     /// <summary>
     /// Adds a second independently addressable key for an existing physical
     /// blob. The alias receives a normal TOC row with the source blob's exact