feat(fuse): implement mkdir and mv with persistence

Add mkdir and mv (rename) FUSE operations to the virtual filesystem:

- mkdir: Create directories that persist across remounts via SQLite
- mv: Move/rename files and directories with database persistence

Changes:
- Add directories table to schema for user-created empty dirs
- Add tree operations: mkdir, rename_file, rename_directory
- Add DB methods for path updates and directory CRUD
- Remove MountOption::RO to allow write syscalls
- Load stored virtual_path from DB instead of regenerating
- Restore user directories on mount from directories table
- Upsert files to DB during origin scan

POSIX compliant: mv fails with ENOENT if parent doesn't exist
(use mkdir first, shell handles -p flag and brace expansion)

crates/musicfs-cache/src/lib.rs

@@ -13,5 +13,6 @@ pub use metadata::MetadataCache;
 pub use patterns::{AccessContext, AccessPattern, PatternError, PatternStore};
 pub use prefetch::{PrefetchConfig, PrefetchEngine, PrefetchHandle};
 pub use tree::{
-    DirNode, FileNode, Inode, RefreshPolicy, TreeBuilder, VirtualNode, VirtualTree, ROOT_INODE,
+    DirNode, FileNode, Inode, RefreshPolicy, RenameError, TreeBuilder, VirtualNode, VirtualTree,
+    ROOT_INODE,
 };