feat: virtual scrolling with chunked buffer for large files
- Add ChunkedBuffer for 64KB chunked file access with dirty-chunk
eviction protection
- Add LineIndex for precise byte-offset-to-line-number mapping
- Refactor IO task system with context cancellation, typed priorities,
and new task types (ReadChunk, BuildLineIndex, StatFile)
- Add ReadFileAt to FileSystem interface (mock + real implementations)
- Integrate virtual scrolling into editor layout
- Add comprehensive tests for chunked buffer eviction, dirty-chunk
safety, and full edit lifecycle
Greg Pomerantz
615957196e