Revert Ratified Evaluation

One-tap revert for an earned-stable promotion (TOS-1982 Phase 1).

Demotes protection_level from 'system_stable' back to 'system_consolidated' - the rung ratification promoted it from. No-op safe: an evaluation that is not currently system_stable is returned unchanged (204), mirroring /unlock's no-op-safe shape. confirmed and confidence are never touched - reverting changes how earned-stable a belief is, not whether or how strongly it is held. Emits its own ELEVATE-kind cognitive_event (action='revert') as the undo journal row, mirroring the promotion side's event in ConsolidationService._ratify_stable_abstractions.

This is a durable veto, not a temporary demotion. The ELEVATE action='revert' event this route writes is also the marker ratification.gather_ratification_candidates checks (RatificationEvidence.was_user_reverted) - once an evaluation has been reverted through this route, the automatic ratification sweep never re-promotes it again, even if it still satisfies every other criterion. The user retains their own promotion paths (/confirm, /lock); only the automatic rung stops overriding this decision.