C2a: mark the header by content, refuse an element twice in a list, route CultMesh single-file through the id rule

A store is written v3 (v5 for a directory) only when a record in it holds a non-empty
element id. CultElementIds.Plan reports that as IdPlan.HoldsIds; the admission stamps it
on the stored record and every store reads it. The descriptor's CarriesElementIds is
deleted. A file already marked on disk stays marked. CultPersistedStoreSnapshot.FormatFor
is the one header decision.

Plan refuses one element object sitting twice in a list at write, naming the record and
list, instead of writing both ids and refusing the store at load.

CultMesh single-file write mints, refuses and marks as a cache write does; the read applies
the load-time plan, so a write and a load refuse the same things.

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

src/GameCult.Caching.MessagePack/DirectoryMessagePackBackingStore.cs

@@ -288,7 +288,7 @@ public sealed class DirectoryMessagePackBackingStore : CacheBackingStore
 
         // A manifest already marked stays marked: pages it names may hold ids this cache cannot read.
         var carriesIds = string.Equals(currentManifest.FormatVersion, IndexedFormatVersionWithIds, StringComparison.Ordinal) ||
-                         keysToWrite.Any(key => Entries.TryGetValue(key, out var written) && written.Descriptor.CarriesElementIds);
+                         keysToWrite.Any(key => Entries.TryGetValue(key, out var written) && written.HoldsIds);
         WriteManifest(targetCatalog, currentIndex.Values
             .OrderBy(record => record.Key, StringComparer.Ordinal)
             .ToArray(), carriesIds);