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/MeshExtractor.cs

@@ -130,8 +130,7 @@ public sealed class MeshExtractor {
             return null;
         }
         catch (OperationCanceledException) {
-            // Ignore
-            return null;
+            throw;
         }
         catch (Exception ex) {
             _logger.LogError(ex, "Error preparing mesh data for 0x{Id:X16}", id);