feat(8.0)!: delete fork/branch/commit/history/versions — superseded by the Db API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Tests: superseded suites deleted; fork/commit blocks excised from shared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

@@ -1,10 +1,10 @@
 /**
- * TestWrappingAdapter: Real COW storage adapter for testing
+ * TestWrappingAdapter: Real blob store adapter for testing
  *
  * This adapter ACTUALLY wraps binary data in JSON (like production storage)
  * to properly test the unwrap logic in BlobStorage.
  *
- * Unlike InMemoryCOWAdapter which stores Buffers directly,
+ * Unlike InMemoryBlobAdapter which stores Buffers directly,
  * this adapter simulates real storage behavior:
  * 1. Compresses with gzip
  * 2. Wraps binary as {_binary: true, data: "base64..."}
@@ -13,14 +13,14 @@
  * This ensures tests exercise the ACTUAL code path that caused v5.10.0 regression.
  */
 
-import { COWStorageAdapter } from '../../src/storage/cow/BlobStorage.js'
+import { BlobStoreAdapter } from '../../src/storage/blobStorage.js'
 import { gzip, gunzip } from 'zlib'
 import { promisify } from 'util'
 
 const gzipAsync = promisify(gzip)
 const gunzipAsync = promisify(gunzip)
 
-export class TestWrappingAdapter implements COWStorageAdapter {
+export class TestWrappingAdapter implements BlobStoreAdapter {
   private storage = new Map<string, Buffer>()
 
   async get(key: string): Promise<any | undefined> {
@@ -39,7 +39,7 @@ export class TestWrappingAdapter implements COWStorageAdapter {
   }
 
   async put(key: string, data: Buffer): Promise<void> {
-    // v6.2.0: Use key-based dispatch (matches baseStorage COW adapter)
+    // Use key-based dispatch (matches the baseStorage blob adapter)
     // NO GUESSING - key format explicitly declares data type
     const obj = key.includes('-meta:') || key.startsWith('ref:')
       ? JSON.parse(data.toString())  // Metadata/refs: ALWAYS JSON