Internal physical storage session with a supervised native Port.

The GenServer serializes storage operations and supervises its linked helper. The helper alone holds flock and writable FDs. A failed or uncertain mutation poisons this instance; no automatic restart/retry is permitted. An embedding supervisor may start this temporary child explicitly. Physical readiness is not Event validation, job insertion, or semantic recovery readiness.

Strict group commits are private Engine-owned sessions: appends update the physical stream in order, one final sync/check closes the group, and the Engine alone withholds all external acknowledgements until that succeeds.