commit_submit(WAIT): n_vcpus-relative admission cap with fast retriable
NOT_READY (admission-cap-only per corrected contract), CLOCK_REALTIME
timedwait loop on the per-LSN + gate predicate, post-timeout predicate
re-check, n_peers<=1 self-quorum through the single advancement site.
Inflight counter accurate under engine lock for the full blocking
interval with XCE_TESTING asserts. 16/16 group tests green unmodified;
full suite at the 3-failure baseline.
9 new cases (8-16): block-until-majority, deadline timeout, standalone
fast path, zero-ACK regression, n_vcpus-relative cap with floor, ACK
progress at saturation, gate as blocking predicate (NOT_READY is
admission-cap-only per corrected contract). Main-thread-only assertions
after joins; 16 tests / 9 RED failures; baseline unchanged.
