Reissue a participant's signing link

Rotates the participant's link credential (a fresh nonce hash replaces the old one, invalidating every outstanding link), appends participant.link_reissued, and emits the notification event. The envelope must be in_progress and the participant pending. No link material appears in the response — delivery happens out of band. Supports the Idempotency-Key header: a replayed completed key returns the envelope's current graph with the original status code and Idempotency-Replayed: true (never re-executing); an in-flight duplicate is rejected 409; reusing a key with a different request is rejected 422.