feat: hunk-level staging — UI + stage/unstage per hunk

[uditdc]

Scope
Builds on the hunk-parsing foundation. Add per-hunk stage/unstage controls in DiffViewer.tsx, wired to git apply --cached with a generated patch.

Acceptance criteria

• Staging a single hunk out of a multi-hunk file results in the correct partial index state (verify via git diff --cached).
• Unstaging a hunk reverses cleanly without affecting other staged hunks in the same file.