Rewrite LineIndex to use sparse sampling (every 256 lines) instead of per-line offsets. Add from_reader() for low-RSS streaming index construction via BufReader fill_buf()/consume(), reducing 5GB file RSS from 5122MB to 3.4MB. Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent) Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>