context: Record mutation cursor coordinator validation

Mark the mutation-cursor runtime coordinator plan complete after the coordinator, snapshot, locking, CAS-retry, taint-recovery, and checkout-ID behaviors passed the recorded validation suite. Preserve the validation evidence and document the intentionally deferred ref reconciliation and external-taint work.

Plan: mutation-cursor-runtime-coordinator
Tasks: AC1-AC13

Co-authored-by: SCE <sce@crocoder.dev>

Author: davidabram