C#: HoldsIds is read off the stored form each time a store writes

A record's HoldsIds was decided by its admission and trusted afterwards, so a
document changed after it was committed, or loaded and changed in place,
flushed under the header its admission-time content needed (Soul P7, P10).

HoldsIds now has one owner, the record's stored form, read when asked:
- a plain record: CultElementIds.Holds over its document, a scan that reads
  and refuses nothing;
- a variant: the same scan over each override value, decoded with the codec
  the record was built with (the cache's, or the attached cache's at load);
- a variant built with no codec (a store that loaded it with no cache) cannot
  see its values and counts as holding an id, so a store never writes an
  unmarked header over ids it could not see.

Cut: the admission path that set HoldsIds on plain records, on variants from
their resolved copies, and on flattened records; the internal setter and its
backing flag; IdPlan.HeldIds and IdPlan.HoldsIds. The Mesh typed write reads
Holds off the document it writes. Admission keeps its own id counting
(IdPlan.Count) for IdsInMemoryOnly.

Tests: an id added to a committed or loaded deck after admission marks a
PushAll v3, and taking it away unmarks it; a variant read without a codec
counts as holding ids while the same record read with one does not.

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

src/GameCult.Mesh/CultMeshSingleFileDocuments.cs

@@ -110,7 +110,7 @@ namespace GameCult.Mesh
                     descriptor.ToCatalogEntry(),
                     storedAt,
                     CultDocumentMessagePackSerialization.SerializeUntyped(document, typeof(TDocument)),
-                    ids.HoldsIds,
+                    CultElementIds.Holds(document),
                     contentKnown: true);
             }
             catch