Segment discovery, naming, and directory open with lock file

[mairinkdev]

Summary

Implement log directory open/create, segment file naming by base offset, and discovery of existing segments. Use a lock file to prevent multiple writers from opening the same log directory.

Scope

• Naming convention: e.g. segment_<base_offset>.log (and later .idx). Document rules (base_offset, ordering).
• Discovery: scan directory for existing segment files; parse base offsets; order segments by base offset.
• Open/create log directory: create if missing; fail or overwrite policy documented.
• Lock file: acquire exclusive lock when opening for write (e.g. write.lock or OS-specific advisory lock); release on drop. Single-writer guarantee.
• Tests: open empty directory (creates nothing or minimal state); open directory with existing segments (discovery returns correct list); lock prevents second writer from opening same path.

Acceptance criteria

• Segment naming and discovery are deterministic and documented.
• Only one writer can hold the log open at a time (enforced via lock file).
• Tests cover empty dir, existing segments, and lock contention behavior.

Suggested commit

feat: implement segment discovery, naming, and directory open