- Fix ChunkedBuffer.Insert to always update file length. - Fix UpdateLineIndexAfterEdit to not shift the first line offset. - Remove redundant ReadFileTask dispatch in HandleBrowserTap. - Add end-to-end test for typing at start of buffer.
- 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
