Summary

• Large gaps (>20 lines) between diff hunks now show directional expand-up/expand-down buttons instead of a single "Expand N unchanged lines" click target
• Each click reveals 20 lines from the chosen direction, inserting a "bridge" hunk rather than destructively merging
• Gaps ≤20 lines retain the single-click expand-all behavior
• Adds buildHunkHeader helper to properly recompute @@ ... @@ headers after expansion, preserving function name suffixes
• Works in both split and unified diff views

Test plan

• 7 new E2E tests in incremental-expand.spec.ts covering directional controls, partial expansion, remaining count update, and both diff modes
• Existing expanded-comments, diff-rendering, auto-expand-gaps tests updated and passing
• All 42 affected tests pass locally

Closes #334

🤖 Generated with Claude Code