feat(8.0)!: delete fork/branch/commit/history/versions — superseded by the Db API

The COW version-control surface (fork, branches, checkout, commit,
getHistory/streamHistory, asOfCommit, brain.versions) is gone, along with
its subsystems: src/versioning/, the COW object store (CommitLog,
CommitObject, RefManager, TreeObject), HistoricalStorageAdapter, and the
TypeAwareHNSWIndex path it kept alive. The Db API (now/transact/asOf/
with/persist/restore) is the one versioning model in 8.0.

Survivors and replacements:
- BlobStorage survives (the VFS stores file content through it), relocated
  to src/storage/blobStorage.ts with binaryDataCodec.ts; its adapter
  interface is now BlobStoreAdapter, slimmed to the consumed surface
  (write/read/has/delete/getMetadata + MIME-aware compression policy).
- brain.migrate() backup branches are replaced by persist-before-migrate:
  MigrateOptions.backupTo persists a hard-link snapshot of the current
  generation before any transform runs; MigrationResult.backupPath reports
  it, and brain.restore(path) brings it back wholesale.
- CLI: cow.ts (fork/branch/checkout/history/migrate) is replaced by
  snapshot.ts — snapshot <path>, restore <path>, history (tx-log),
  generation.
- New public read API: brain.transactionLog({limit}) exposes the reified
  tx-log (generation/timestamp/meta, newest first) that backs the CLI
  history command; TxLogEntry is exported.

Tests: superseded suites deleted; fork/commit blocks excised from shared
suites; BlobStorage tests relocated + reworked against the slimmed store;
migration tests now prove the backupTo snapshot/restore round trip; new
transactionLog coverage in db-mvcc.

src/storage/adapters/memoryStorage.ts

@@ -362,8 +362,13 @@ export class MemoryStorage extends BaseStorage {
     this.statisticsCache = null
     this.statisticsModified = false
 
+    // Drop the BlobStorage instance — its LRU cache holds deleted blobs.
+    // initializeBlobStorage() re-creates it (brain.clear() does this before
+    // the VFS is rebuilt).
+    this.blobStorage = undefined
+
     // Clear write-through cache (inherited from BaseStorage)
-    // Without this, readWithInheritance() would return stale cached data
+    // Without this, readCanonicalObject() would return stale cached data
     // after clear(), causing "ghost" entities to appear
     this.clearWriteCache()
   }
@@ -390,17 +395,6 @@ export class MemoryStorage extends BaseStorage {
     }
   }
 
-  /**
-   * Check if COW has been explicitly disabled via clear()
-   * No-op for MemoryStorage (doesn't persist)
-   * @returns Always false (marker doesn't persist in memory)
-   * @protected
-   */
-  /**
-   * Removed checkClearMarker() and createClearMarker() methods
-   * COW is now always enabled - marker files are no longer used
-   */
-
   /**
    * Save statistics data to storage
    * @param statistics The statistics data to save