fix(vfs): resolve two critical VFS bugs causing directory listing corruption

Bug 1: verbCountsByType optimization skipping requested verb types
- After restart, stale statistics could cause VerbType.Contains to be skipped
- readdir() would return empty/incomplete results
- Fixed by never skipping verb types explicitly requested in filter
- Added fast path for sourceId + verbType combo (common VFS pattern)
- Save statistics on first entity of each type (not just every 100th)

Bug 2: UnifiedCache not invalidated on path deletion
- rmdir() cleared local caches but NOT the global UnifiedCache
- When folder recreated, resolve() returned stale entity ID
- Caused "Source entity not found" errors
- Fixed by adding deleteByPrefix() to UnifiedCache
- Fixed invalidatePath() to also clear UnifiedCache entries

Files modified:
- src/storage/baseStorage.ts (verbCountsByType fix + fast path)
- src/utils/unifiedCache.ts (deleteByPrefix method)
- src/vfs/PathResolver.ts (cache invalidation fix)

Tests added:
- tests/unit/storage/vfs-mkdir-bug.test.ts (7 tests)

Reported by: Soulcraft Workshop team

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/utils/unifiedCache.ts

@@ -415,6 +415,24 @@ export class UnifiedCache {
     return false
   }
 
+  /**
+   * Delete all items with keys starting with the given prefix
+   * v6.2.9: Added for VFS cache invalidation (fixes stale parent ID bug)
+   * @param prefix - The key prefix to match
+   * @returns Number of items deleted
+   */
+  deleteByPrefix(prefix: string): number {
+    let deleted = 0
+    for (const [key, item] of this.cache) {
+      if (key.startsWith(prefix)) {
+        this.currentSize -= item.size
+        this.cache.delete(key)
+        deleted++
+      }
+    }
+    return deleted
+  }
+
   /**
    * Clear cache or specific type
    */