feat(8.0)!: delete fork/branch/commit/history/versions — superseded by the Db API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Tests: superseded suites deleted; fork/commit blocks excised from shared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

@@ -912,7 +912,7 @@ export class FileSystemStorage extends BaseStorage {
         if (entry.isFile()) {
           // Handle multiple compression formats for broad compatibility
           // - .json.gz: Standard entity/metadata files (JSON compressed)
-          // - .gz: COW files (refs, blobs, commits - raw compressed)
+          // - .gz: Raw compressed payloads (e.g. blob-store binary values)
           // - .json: Uncompressed JSON files
           if (entry.name.endsWith('.json.gz')) {
             // Strip .gz extension and add the .json path
@@ -923,7 +923,7 @@ export class FileSystemStorage extends BaseStorage {
               seen.add(normalizedPath)
             }
           } else if (entry.name.endsWith('.gz')) {
-            // COW files stored as .gz (not .json.gz)
+            // Raw payloads stored as .gz (not .json.gz)
             // Strip .gz extension and return path
             const normalizedName = entry.name.slice(0, -3) // Remove .gz
             const normalizedPath = path.join(prefix, normalizedName)
@@ -1387,12 +1387,10 @@ export class FileSystemStorage extends BaseStorage {
       }
     }
 
-    // Clear the entire branches/ directory (branch-based storage)
-    // Bug fix: Data is stored in branches/main/entities/, not just entities/
-    // The branch-based structure was introduced for COW support
-    const branchesDir = path.join(this.rootDir, 'branches')
-    if (await this.directoryExists(branchesDir)) {
-      await removeDirectoryContents(branchesDir)
+    // Clear the canonical entity/verb data area
+    const entitiesDir = path.join(this.rootDir, 'entities')
+    if (await this.directoryExists(entitiesDir)) {
+      await removeDirectoryContents(entitiesDir)
     }
 
     // Remove all files in both system directories
@@ -1401,19 +1399,16 @@ export class FileSystemStorage extends BaseStorage {
       await removeDirectoryContents(this.indexDir)
     }
 
-    // Remove COW (copy-on-write) version control data
-    // This directory stores all git-like versioning data (commits, trees, blobs, refs)
-    // Must be deleted to fully clear all data including version history
-    const cowDir = path.join(this.rootDir, '_cow')
-    if (await this.directoryExists(cowDir)) {
-      // Delete the entire _cow/ directory (not just contents)
-      await fs.promises.rm(cowDir, { recursive: true, force: true })
+    // Remove the content-addressed blob store (VFS file content)
+    const casDir = path.join(this.rootDir, '_cas')
+    if (await this.directoryExists(casDir)) {
+      // Delete the entire _cas/ directory (not just contents)
+      await fs.promises.rm(casDir, { recursive: true, force: true })
 
-      // Reset COW managers (but don't disable COW - it's always enabled)
-      // COW will re-initialize automatically on next use
-      this.refManager = undefined
+      // 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
-      this.commitLog = undefined
     }
 
     // Clear the statistics cache
@@ -1426,7 +1421,7 @@ export class FileSystemStorage extends BaseStorage {
     ;(this as any).totalVerbCount = 0
 
     // 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()
   }
@@ -1457,17 +1452,6 @@ export class FileSystemStorage extends BaseStorage {
     return result
   }
 
-  /**
-   * Check if COW has been explicitly disabled via clear()
-   * Fixes bug where clear() doesn't persist across instance restarts
-   * @returns true if marker file exists, false otherwise
-   * @protected
-   */
-  /**
-   * Removed checkClearMarker() and createClearMarker() methods
-   * COW is now always enabled - marker files are no longer used
-   */
-
   /**
    * Get information about storage usage and capacity
    */