Background threads (spawn_indexer, spawn_visual_height_rebuild) previously
held mmap during entire file scan, risking SIGBUS if file was truncated
externally. Now uses BufReader streaming scan with mmap created only
after scan completes, plus stat validation.
Changes:
- spawn_indexer: replace mmap scan with BufReader fill_buf/consume loop,
create mmap post-scan with fd stat validation
- spawn_visual_height_rebuild: replace mmap/FileReader with sequential
BufReader scan, discard results on line count mismatch
- FileReader::open/reload/update_for_append: add stat-after-mmap check
- LineIndex: make fields pub(crate) for direct construction from scan loop
- Add 3 regression tests for truncation scenarios
1350f659fa