Route each document type to one home store; attach hydrates; commit explicit batches

Cut 3 of the CultCache migration (Aetheria docs/cultcache-migration-cut.md, 2.1
and 2.2). One commit, because routing, the batch that replaces the ambient
transaction, and conditional commit share one admission path.

Deleted first:
- globals: InitializeGlobals, MaterializeMissingGlobals, the two-argument
  constructor, the initializeGlobals plumbing in CultCacheMessagePack,
  ContainsDurableRecord (cache and directory store)
- replication: the push-to-every-store loops and the attach-time push of
  existing records
- typeof(T) as the schema owner in AddAsync/UpsertAsync and in
  CultNetDatabase.PutAsync/PutPredictedAsync
- the ambient transaction: ExecuteTransactionAsync x2, CultCacheTransaction,
  CommitTransaction, VisibleStoredDocuments, the AsyncLocal, the SemaphoreSlim
  gate, and every ambient check
- CultCacheOpenOptions.PullOnOpen; OpenAsync is Task.FromResult(Create(...))
- DirectoryMessagePackBackingStore.ResolveLegacyUncataloguedRecordCatalog: the
  catalog precommit (fe55bff) is an ancestor of the v4 page format (eea8248),
  so no v4 manifest can reach it
- CultCache.DeleteAsync<T>; its one caller, CultNetDatabaseSubscriptionClient,
  called it by reflection and now calls Remove(key)

Added:
- AddBackingStore(store, params Type[] homes): attach validates (clean, not
  already attached, one untyped store, no claimed home type, no admitted
  record changes home) and then pulls under the gate
- Home(type): most derived routed home, else the untyped store
- Admit: every add and remove, from a write, a batch or a load, validates
  (home, read-only, loaded-from-home, global singleton), lands durably, then
  applies and publishes; a pull hands its whole delta over in one call, so a
  refused record leaves the store and the cache unchanged
- read-only stores (CacheBackingStore(readOnly), both store constructors,
  CultCacheOpenOptions.ReadOnly); flush skips them and writes only dirty stores
- assignable GetGlobal<T>/GetByName<T>/GetByIndex<T>/Watch<T>; more than one
  candidate throws naming them
- Commit(Action<CultCacheBatch>) returning bool, TryCommit returning
  CultCommitOutcome, CultCacheBatch (Upsert, Remove, Expect, ExpectUnchanged,
  sealed after commit), CultCommitRequest; CacheBackingStore.CommitBatch
  evaluates conditions under the store's exclusive lock
- single-file store: <path>.lock sidecar for CommitBatch and PushAll; a clean
  store commits onto the file as it is, so records it never loaded survive
- storedAt minted in one place and bumped a tick when not later than the record
  it replaces

Consumer adjustments in this repo: Mesh streaming tests drop PullOnOpen; the
Studio bridge stops reflecting on it; BackingStoreTests' global test creates
its global explicitly and its transaction tests are rewritten against Commit
(the two that proved ambient hydration rules are deleted); the pre-Cut 2
refusal test expects the attach to throw.

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

src/GameCult.Networking/CultNetDatabase.cs

@@ -805,7 +805,7 @@ namespace GameCult.Networking
             ThrowIfDisposed();
             if (document == null) throw new ArgumentNullException(nameof(document));
 
-            var descriptor = _cache.Registry.GetRequired<T>();
+            var descriptor = _cache.Registry.GetRequired(document.GetType());
             var shard = ResolveShardInternal(descriptor, key);
             EnsurePrimary(shard, descriptor.SchemaId, key);
 
@@ -840,7 +840,7 @@ namespace GameCult.Networking
             ThrowIfDisposed();
             if (document == null) throw new ArgumentNullException(nameof(document));
 
-            var descriptor = _cache.Registry.GetRequired<T>();
+            var descriptor = _cache.Registry.GetRequired(document.GetType());
             var shard = ResolveShardInternal(descriptor, key);
             EnsureClientAuthority(descriptor, key);