feat(8.0)!: delete fork/branch/commit/history/versions — superseded by the Db API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Tests: superseded suites deleted; fork/commit blocks excised from shared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

@@ -152,9 +152,8 @@ export abstract class BaseStorageAdapter implements StorageAdapter {
   //   "graph-lsm/source/sstable-123" "<root>/_blobs/graph-lsm/source/sstable-123.bin"
   //
   // i.e. the key's "/"-separated segments are nested under a `_blobs/` prefix and
-  // suffixed with `.bin`. Blobs are deliberately NOT branch-scoped (COW): they
-  // are immutable, content-addressed segments managed by their producer,
-  // mirroring how COW metadata under `_cow/` is also kept global.
+  // suffixed with `.bin`. Blobs are immutable, content-addressed segments
+  // managed by their producer.
 
   /**
    * Persist a raw binary blob under `key`, writing the bytes verbatim (no JSON