Implement truthful change classification and diff review

[Traviinam]

Summary

• classify conflicts, staged changes, working-tree changes, and untracked files truthfully
• add lazy bounded diff review with unified and side-by-side presentation
• preserve opaque file targeting and provide safe degraded native actions
• cover renames, deletions, conflicts, unusual paths, and non-renderable content

Testing

• npm test (35 files, 218 tests)
• npm run build
• npm run lint
• npm run format:check

Closes #9