Right now Expand N unchanged lines is one-way because expanding mutates the hunk structure. Add a corresponding collapse action so reviewers can re-fold expanded unchanged sections.

Requirements:

• Support both leading folded context and gaps between hunks.
• Restore the original hunk boundaries when collapsed.
• Work in both split and unified diff views.
• Preserve comment behavior and avoid breaking open comment forms on expanded lines.

Implementation note:

• Prefer storing expand/collapse as UI state instead of destructively merging file.diffHunks.