Covers IntersectionObserver placeholder rendering for 300-file diffs,
single-file bypass, sidebar ref registration, expandAll batch fetching
(25 files → 3 batches of 10), and all FileCard lazy-load states:
default collapsed, loading, success with HunkRows, error+retry, binary,
no-hunks, detail-prop pre-expanded, and collapse/re-expand UX.
Cherry-picks viewport virtualization implementation commit (f804cb19)
onto this branch so the tests run against the actual new code.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
