CultCache: every backing store marks the records it rewrites, through a protected CacheBackingStore.MarkWritten

The directory store's write of a generation marks each page it wrote once the
manifest names it, so a Push+PushAll of a loaded drifted entry no longer leaves
its persisted schema id stale. The single-file store marks through the same
member, and a store outside GameCult.Caching now has a supported way to mark
its own rewrite. The getter's doc names how a store records a rewrite.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/GameCult.Caching.MessagePack/DirectoryMessagePackBackingStore.cs

@@ -263,6 +263,7 @@ public sealed class DirectoryMessagePackBackingStore : CacheBackingStore
         var keysToWrite = _dirtyKeys.Keys
             .OrderBy(value => value, StringComparer.Ordinal)
             .ToArray();
+        var written = new List<CultStoredDocument>(keysToWrite.Length);
         foreach (var key in keysToWrite)
         {
             if (!Entries.TryGetValue(key, out var stored))
@@ -281,11 +282,15 @@ public sealed class DirectoryMessagePackBackingStore : CacheBackingStore
             WriteFileAtomically(
                 ContentAddressedRecordPath(indexRecord),
                 pagePayload);
+            written.Add(stored);
         }
 
         WriteManifest(targetCatalog, currentIndex.Values
             .OrderBy(record => record.Key, StringComparer.Ordinal)
             .ToArray());
+        // The manifest names the new pages: each was written under its own descriptor.
+        foreach (var stored in written)
+            MarkWritten(stored);
 
         DeleteUnreferencedRecordPages(currentIndex.Values);