feat(8.0)!: delete fork/branch/commit/history/versions — superseded by the Db API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Tests: superseded suites deleted; fork/commit blocks excised from shared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.

tests/integration/metadata-only-comprehensive.test.ts

@@ -291,40 +291,6 @@ describe('Metadata-Only Comprehensive Integration (v5.11.1)', () => {
     })
   })
 
-  describe('COW and Fork', () => {
-    it('should work with metadata-only in forks', async () => {
-      const brain = new Brainy({ requireSubtype: false,
-        storage: { adapter: 'memory' },
-        silent: true
-      })
-
-      await brain.init()
-
-      const id = await brain.add({
-        data: 'Original',
-        type: NounType.Document,
-        metadata: { version: 1 }
-      })
-
-      await brain.commit({ message: 'Initial commit' })
-
-      const fork = await brain.fork('test-branch')
-
-      // Metadata-only in fork
-      const entity = await fork.get(id)
-      expect(entity).toBeTruthy()
-      expect(entity!.metadata.version).toBe(1)
-      expect(entity!.vector).toEqual([])
-
-      // Full entity in fork
-      const full = await fork.get(id, { includeVectors: true })
-      expect(full!.vector.length).toBe(384)
-
-      await brain.close()
-      await fork.close()
-    })
-  })
-
   describe('Performance Verification', () => {
     it('metadata-only should be significantly faster', async () => {
       const brain = new Brainy({ requireSubtype: false,