feat(scheduling): add strict FIFO lanes

[psteinroe]

Add opt-in strict FIFO processing with one persistent owner per (queue, task). FIFO tasks ignore priority and retain ownership across retries, sleeps, child waits, and recovery until a guarded terminal transition releases the lane.

Future executions that have never started do not block runnable work. FIFO is mutually exclusive with soft task/group concurrency at both the type and database levels.

This also fixes live-queue refill behavior in the concurrent mapper so newly available work cannot deadlock behind an active task.

Stacked on #17.

Closes #12

[psteinroe]

Closing because the strict FIFO design has been intentionally replaced with PostgreSQL-style best-effort ordering by priority, run time, creation time, and ID.