Add CpSharedL2PageAllocator.mark_object_committed(object_key) -- the B1 commit path that marks an object committed directly, bypassing the per-(rank,layer,payload) commit_layer/_has_full_commit quorum. Under B1 the all-ranks-done consensus is the writing_check ReduceOp.MIN frontier (the same barrier that releases the node write lock), so every rank calls this with the same object_key at the MIN commit point and the committed set transitions rank-uniformly (feeds placement_digest).
Resolves opus-review MF4: split_committed_object stays coherent for a B1-committed object (no per-layer commit facts) -- it adds children to _committed_objects and the empty _commits_by_object is harmless because B1 never calls _has_full_commit.
7 unit tests (pure-Python): commit-without-quorum, idempotent, unknown-raises, stat bump, digest reflects committed set + lockstep equality, split-of-marked-object coherent, release-after-mark frees pages. Full suite 88/88 green on g0033.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
leavelet
50bc923ad6