Off-lock segment flush

Flushes no longer hold the write lock for the encode — only for a brief seal and commit, mirroring merges. Worst-case concurrent search latency during ingest: 233 ms → 106 ms, and much less variable run-to-run

What's Changed

• Feature/disk segment writer by @adikeshri in #10

Full Changelog: v4.1.0...v4.2.0