fix(app): stabilize canvas diff rendering

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.

packages/app/src/review/index.ts

@@ -30,6 +30,7 @@ export {
 export {
   getInlineReviewThreadViewportStyle,
   groupInlineReviewCommentsByTarget,
+  InlineReviewAddButton,
   InlineReviewEditor,
   InlineReviewGutterCell,
   InlineReviewThread,