Fixes what the job-status writers do today, so routing them through a single
transition boundary has an oracle instead of a promise. Same ordering that paid
off in phase 1.
X-02 states the claim path's real contract rather than the one the plan first
assumed: pick_pending filters on pending under the lock, so it is already a
correct swap. It duplicates the chokepoint, it does not bypass its safety. The
test pins the properties that must survive routing -- only pending is claimed,
only one session wins, and a session never sees another's work.
Every test is parametrized over MAM_ENGINE_CAS so the legacy body and the engine
body must agree.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UrDNJkHmm18uu7n7JwV5Qd
