feat(workpool): deliver one aggregate result and selected default

Install OQ2 keep_alive as the omission default while leaving fresh selectable.
Close/cancel/inspect complete the pool surface: close returns (pool_id, generation)
immediately and delivers one input-order aggregate over existing parent-session
notification routing; cancel stops unstarted work and aborts owned workers;
inspect is a nonblocking persisted read that survives kernel reset.

Also land the row-12 measurement harness and frozen 12-item batch used to choose
the default (xai/grok-4.6 AB/BA, conclusive keep_alive).