Add 7 new tests covering virtual list behavior with large datasets:
- 9000-entry rendering without crash
- Items rendered via Virtuoso mock
- Search filtering on large dataset
- Sorting correctness with large dataset
- Section group filtering with mixed types
- Selection highlighting in virtualized list
- Click handler on virtualized items
Add design/performance-note-list.pen with 4 frames:
1. Default state — 9000+ notes with scrollbar
2. Scrolled mid-list — items 4500+
3. Search filtering — active query narrowing results
4. Empty search result — no matching notes
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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