Critical performance fix
- Fix copy-on-write behavior that made lazy sector-chain indexing quadratic
- Keep references to the cached chain state while extending it
- Track the chain tail directly instead of copying the growing sector list
- Restore linear scaling for first reads of large streams
- Extend the benchmark to report cold complete-stream extraction before cache warm-up
A 4,366-sector XLS stream measured approximately 7.4x faster with this fix, with identical extracted bytes. The public API is unchanged.
All formatting, static analysis, and PHPUnit checks pass.