## Context\n\nDeep nesting (depth > 3 with 10+ children) causes performance cliff.\n\n## Goal\n\nImplement nested virtualization for smooth performance with deeply nested structures.\n\n## Implementation\n\n- File: Create src/outliner/VirtualNestedBlockList.tsx\n- Trigger virtualization when depth > 3 with 10+ children\n- Effort: 4h\n\n## Impact\n\nSmooth performance for deeply nested structures.\n\n## References\n\n- ARCHITECTURE_REVIEW.md Section 9, Wave 2, Item 6\n- Issue H1 in Issue Catalog
