C# core: variant records, top-level Set overrides, resolution in the admitting hold

A variant record stores [baseKey, overrides[]] in record slot 4 and an empty
payload; the cache resolves it against its base inside the hold that admits it or
any record on its base chain, and re-resolves dependents in the same Apply, so a
base change reaches its variants with contiguous sequences and no stamp. Plain
Upsert at a variant key is refused (Flatten is the explicit way); cycle, missing
base, different concrete type, evicted base with survivors and a name that
shadows the base are refused naming the keys. Overrides of a dropped slot or an
undecodable value are ignored with a warning and reported as compatible drift.

The store writes cultcache.store.v2 only when it holds a variant and reads v1 and
v2. The codec pair CultInspectorModel took becomes CultCodec, shared with the
cache, which needs it to clone documents and overlay override values. The
directory store, CultMesh single-file reads and CultNet row building refuse a
variant by name until their own cuts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/GameCult.Caching.MessagePack/DirectoryMessagePackBackingStore.cs

@@ -135,9 +135,18 @@ public sealed class DirectoryMessagePackBackingStore : CacheBackingStore
         Trace("publish");
     }
 
+    // Variants wait for the directory store's own cut; until then it refuses one rather than persist a resolved view as a plain page.
+    private static void RefuseVariant(CultStoredDocument entry)
+    {
+        if (entry.Variant != null)
+            throw new NotSupportedException(
+                $"Record '{entry.Key.Value}' is a variant of '{entry.Variant.BaseKey}'; the directory store does not hold document variants yet.");
+    }
+
     public override void Push(CultStoredDocument entry)
     {
         ThrowIfReadOnly();
+        RefuseVariant(entry);
         Held(() =>
         {
             Entries[entry.Key.Value] = entry;
@@ -162,6 +171,8 @@ public sealed class DirectoryMessagePackBackingStore : CacheBackingStore
     public override CultCommitOutcome CommitBatch(CultCommitRequest request, bool wait)
     {
         ThrowIfReadOnly();
+        foreach (var entry in request.Upserts)
+            RefuseVariant(entry);
         return Held(() =>
         {
             Directory.CreateDirectory(_manifestFile.DirectoryName!);