Implements core startup infrastructure:
- AppConfig for data directory paths
- ensure_data_dir() to create ~/.local/share/mc/
- verify_cli_dependencies() async CLI availability check
- load_with_migration() for state file loading with migration support
- init() main entry point coordinating lock acquisition and setup
- StartupWarning enum for non-fatal issues (missing CLIs, state reset)
- InitError enum with conversion to McError
Also exposes Bridge::with_data_dir() publicly (was test-only).
Includes 11 tests covering directory creation, lock acquisition,
lock contention, corrupt file handling, and config paths.
bd-3jh
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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