* perf(app): keep large diffs responsive
Render file headers on the canvas and bound expensive text shaping to the active viewport while preserving sticky, hover, and horizontal-scroll behavior. Let native scroll cancellation arbitrate header presses so finger scrolling cannot expand files.
* test(app): assert canvas diff header accessibility
* test(app): assert canvas diff header semantics
Keep diff headers and painted content on the same scroll clock while preserving native scrolling and independent per-file horizontal offsets.
Complete review, selection, typography, tree, and palette parity without returning line rendering to React.
Mohamed Boudra