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)

docs/v2/features/mkdir.md

@@ -0,0 +1,105 @@
+**Date**: 2026-05-17
+**Status**: Shipped
+
+# Feature: Create Directory (mkdir)
+
+## Overview
+
+MusicFS supports creating directories in the virtual filesystem. This enables organizing files into custom folder structures beyond the auto-generated metadata-based layout.
+
+## Behavior
+
+### Basic Usage
+
+```bash
+mkdir "/mnt/music/New Artist"
+mkdir "/mnt/music/New Artist/New Album"
+```
+
+- Creates empty directory at specified path
+- Parent directory must exist
+- Standard POSIX semantics
+
+### Nested Directories
+
+```bash
+# This works (shell handles -p)
+mkdir -p "/mnt/music/A/B/C"
+
+# Equivalent to:
+mkdir "/mnt/music/A"
+mkdir "/mnt/music/A/B"
+mkdir "/mnt/music/A/B/C"
+```
+
+The `-p` flag is handled by the shell, which makes multiple `mkdir` syscalls.
+
+### Brace Expansion
+
+```bash
+# Shell expands this to multiple mkdir calls
+mkdir "/mnt/music/Artist/{Album1,Album2,Album3}"
+
+# Equivalent to:
+mkdir "/mnt/music/Artist/Album1"
+mkdir "/mnt/music/Artist/Album2"
+mkdir "/mnt/music/Artist/Album3"
+```
+
+Brace expansion is shell functionality, not filesystem.
+
+## Error Codes
+
+| Condition | Error |
+|-----------|-------|
+| Parent doesn't exist | `ENOENT` |
+| Path already exists | `EEXIST` |
+
+## Persistence
+
+**Empty directories persist across remounts.**
+
+- User-created directories are stored in the `directories` table
+- On mount, directories are restored from database
+- Directories survive even when empty
+
+## Use Cases
+
+### Organizing Downloads
+
+```bash
+# Create structure
+mkdir "/mnt/music/Unsorted"
+mkdir "/mnt/music/Unsorted/2026"
+
+# Move untagged files
+mv "/mnt/music/Unknown Artist/Unknown Album/"*.flac "/mnt/music/Unsorted/2026/"
+```
+
+### Custom Collections
+
+```bash
+# Create playlist-like structure
+mkdir "/mnt/music/_Playlists"
+mkdir "/mnt/music/_Playlists/Road Trip"
+
+# Move tracks (they'll still be in original location too - wait, no they won't)
+# Note: mv moves, doesn't copy
+```
+
+## Implementation
+
+| Component | File |
+|-----------|------|
+| Tree | `crates/musicfs-cache/src/tree.rs` |
+| FUSE | `crates/musicfs-fuse/src/filesystem.rs` |
+
+### Key Functions
+
+- `VirtualTree::mkdir()` - Create directory node in tree
+- `Filesystem::mkdir()` - FUSE operation handler
+
+## Limitations
+
+- **No permissions**: Mode/umask parameters are ignored (always 0755)
+- **No ownership**: UID/GID set to mounting user