Add a centralized ui module with Arc's visual identity: colored commit
IDs (magenta), bookmarks (cyan), tags (yellow), status symbols, and
diff highlighting. Update all command output and tests accordingly.
- arc log: shows commit history newest-first with optional range support
- arc show: displays commit details by bookmark/tag/commit-id/prefix
- arc history: blame-style line attribution using Myers diff algorithm
- New resolve module for target resolution (bookmark → tag → commit prefix)
- New inspect module with timestamp formatting (no extra deps)
- Tests for all three commands (17 new tests, 82 total)
