## Summary

Adds three sorting modes for the project panel to give users more
control over how files and directories are displayed:

- **`directories_first`** (default): Current behaviour - directories grouped before files
- **`interleaved`**: Files and directories sorted together alphabetically
- **`macos_like`**: Like interleaved, but with case-insensitive sorting (Finder-style)

 ## Motivation

Users coming from different editors and file managers have different
expectations for file sorting. Some prefer directories grouped at the
top (traditional), while others prefer the macOS Finder-style
interleaved sorting where "Apple1/", "apple2.tsx" and "Apple3/" appear
alphabetically mixed together.

 ## Changes

 ### Core Implementation

- Added `ProjectPanelSortMode` enum to settings with three variants
- Implemented `compare_rel_paths_interleaved()` for mixed file/directory sorting
- Implemented `compare_rel_paths_macos_like()` for case-insensitive interleaved sorting
- Refactored sorting functions to accept a mode parameter while maintaining backward compatibility
- Added `sort_mode` field to `ProjectPanelSettings` with `directories_first` as default

 ### User Experience

- Settings UI integration with dropdown selector in Panels section
- Real-time panel updates when sort mode changes
- Documentation added to visual-customization.md
- All three modes use natural sorting for numbers (e.g., "file2" < "file10")

 ### Testing & Performance

- Comprehensive test coverage including edge cases (basename collisions, nested paths, stability)
- Benchmark suite to measure performance across all three modes (interleaved is fastest, followed by macos-like, followed by the current implementation)
- Fixed hard-coded path in benchmark to use `CARGO_MANIFEST_DIR`

 ## Technical Details

**Sorting Implementations:**

1. **`directories_first`**: Uses existing `compare_rel_paths()` - directories have inherent priority, then natural sort within each group

2. **`interleaved`**: New `compare_rel_paths_interleaved()` - removes directory/file bias, compares by natural sort with deterministic full-path tie-breaking for entries like `foo/` vs `foo.rs`

3. **`macos_like`**: New `compare_rel_paths_macos_like()` - case-insensitive variant of interleaved using `natural_sort_case_insensitive()` helper

**Key Design Decisions:**

- Maintained backward compatibility with legacy `sort_worktree_entries()` functions
- Used mode-aware wrappers (`sort_worktree_entries_with_mode()`) for new code
- Settings observer triggers `update_visible_entries()` when sort mode changes
- All sorting happens in-place for performance

in initializer of `workspace::ProjectPanelSettingsContent`

Noticed a bug after using this for a while in that the children of an
expanded directory could get separated from the parent directory node
by other similiarly named files.