Retire the last test fixture for deleted surface; update the tests README

SchemaStampedTestEntry was only used by the v1 missing-catalog load test
deleted with the legacy directory formats. The Caching tests README now
describes what the suite covers after the subtraction, including the ambient
transaction tests kept until Cut 3 and the red store-routing contract tests.

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

tests/GameCult.Caching.Tests/README.md

@@ -1,15 +1,32 @@
 # GameCult.Caching.Tests
 
 `GameCult.Caching.Tests` contains NUnit tests for the caching library and its
-single-file MessagePack persistence path.
+MessagePack persistence stores.
 
 ## Scope
 
 The tests currently cover:
 
-- `SingleFileMessagePackBackingStore` round-trips
-- explicit Cult document payload codecs
-- hand-written MessagePack store snapshot/record/catalog serialization
+- `SingleFileMessagePackBackingStore` round-trips, external pulls, and the
+  dirty-pull guard
+- `DirectoryMessagePackBackingStore` with the
+  `cultcache.store.v4.directory-content-addressed-pages` manifest: page writes,
+  clean flushes, concurrent writers under the commit lease, external deltas,
+  unflushed local keys, orphaned pages, and observers running after the lease
+  is released
+- `CultCacheMessagePack.OpenAsync`, persisted globals, dirty state, and flush on
+  dispose
+- cache lookups (`Get`, `GetAll`, runtime-type upserts)
+- the ambient cache transaction (`ExecuteTransactionAsync`), until Cut 3
+  replaces it with the explicit batch commit
+- explicit Cult document payload codecs, generated metadata, canonical schema
+  fixtures, and schema drift reports
+- hand-written MessagePack snapshot and record serialization
+- `StoreRoutingTests`: the Cut 1 store-routing contract, red until Cut 3
+
+Selective hydration, the directory store's stage probes, pre-v4 directory
+formats, and the SoA table are no longer in the tree; the SoA table is parked
+at `parked/cultcache-soa` (`docs/parked-features.md`).
 
 ## Run