perf(content): add typed prepared asset source

Introduce explicit loaded, missing, and corrupt outcomes over typed pak keys, validate package/DAT identity at open, add zero-I/O TOC probes, preserve cancellation, and route the package path through RuntimeOptions. Production injection follows in the next checkpoint.

src/AcDream.Content/Pak/PakReadStatus.cs

@@ -0,0 +1,24 @@
+namespace AcDream.Content.Pak;
+
+/// <summary>
+/// TOC-only state for one exact typed pak key. <see cref="Available"/> means
+/// the immutable TOC row exists and has a structurally valid file range; CRC
+/// and payload structure are deliberately not faulted in by a probe.
+/// </summary>
+public enum PakEntryState
+{
+    Missing,
+    Available,
+    Corrupt,
+}
+
+/// <summary>
+/// Result of reading and deserializing one pak payload. This preserves the
+/// distinction between an absent key and a present-but-damaged external file.
+/// </summary>
+public enum PakObjectReadStatus
+{
+    Missing,
+    Loaded,
+    Corrupt,
+}

src/AcDream.Content/Pak/PakReader.cs

@@ -112,21 +112,45 @@ public sealed class PakReader : IDisposable {
         return VerdictFor(index) == 1;
     }
 
+    /// <summary>
+    /// Probes the immutable TOC only. No payload bytes are copied, CRC-checked,
+    /// or deserialized. A structurally invalid entry is reported as corrupt;
+    /// a CRC or payload-structure fault becomes visible after the first read.
+    /// </summary>
+    public PakEntryState ProbeEntry(ulong key) {
+        int index = BinarySearch(key);
+        if (index < 0)
+            return PakEntryState.Missing;
+        return _entryVerdictByTocIndex.TryGetValue(index, out int verdict)
+            && verdict == 0
+                ? PakEntryState.Corrupt
+                : PakEntryState.Available;
+    }
+
     /// <summary>
     /// Reads and deserializes the <see cref="ObjectMeshData"/> stored under
     /// <paramref name="key"/>. Returns false (data null) if the key is absent
     /// OR the blob fails any tripwire (bounds, CRC, structural deserialization
     /// failure) — logged once per entry, per the corrupt-=-missing contract.
     /// </summary>
-    public bool TryReadObjectMeshData(ulong key, out ObjectMeshData? data) {
+    public bool TryReadObjectMeshData(ulong key, out ObjectMeshData? data) =>
+        ReadObjectMeshData(key, out data) == PakObjectReadStatus.Loaded;
+
+    /// <summary>
+    /// Reads one payload while preserving absent-versus-corrupt state for the
+    /// production prepared-asset boundary.
+    /// </summary>
+    public PakObjectReadStatus ReadObjectMeshData(
+        ulong key,
+        out ObjectMeshData? data) {
         data = null;
         int index = BinarySearch(key);
-        if (index < 0) return false;
+        if (index < 0) return PakObjectReadStatus.Missing;
 
         // Previously judged bad (bounds at open, or an earlier CRC/structure
         // failure): missing, no re-read, no re-log.
         bool judged = _entryVerdictByTocIndex.TryGetValue(index, out var verdict);
-        if (judged && verdict == 0) return false;
+        if (judged && verdict == 0) return PakObjectReadStatus.Corrupt;
 
         // Single pass (review finding 4): ONE copy out of the map; CRC and
         // deserialization both run over this same buffer.
@@ -139,14 +163,14 @@ public sealed class PakReader : IDisposable {
             if (actualCrc != entry.Crc32) {
                 _entryVerdictByTocIndex[index] = 0;
                 LogCorruptionOnce(index, $"crc mismatch (expected 0x{entry.Crc32:X8}, got 0x{actualCrc:X8})");
-                return false;
+                return PakObjectReadStatus.Corrupt;
             }
             _entryVerdictByTocIndex[index] = 1;
         }
 
         try {
             data = ObjectMeshDataSerializer.Read(bytes);
-            return true;
+            return PakObjectReadStatus.Loaded;
         }
         catch (Exception ex) {
             // Structurally malformed blob behind a valid CRC (bake-side bug or
@@ -155,7 +179,7 @@ public sealed class PakReader : IDisposable {
             data = null;
             _entryVerdictByTocIndex[index] = 0;
             LogCorruptionOnce(index, $"deserialization failed despite matching CRC: {ex.GetType().Name}: {ex.Message}");
-            return false;
+            return PakObjectReadStatus.Corrupt;
         }
     }