feat: add optional JPEG XL output

[deeplook]

Summary

• Add optional lossless JPEG XL output to map and diff, plus static git/hg snapshots and watch snapshots.
• Add the pillow-jxl-plugin extra and document installation and supported commands.
• Add encoder and CLI tests, and update highlight tests to avoid Pillow's deprecated pixel accessor.

Validation

• Focused JXL, watch, and highlight tests: 50 passed.
• Pre-commit checks, including Ruff and mypy, passed.