Abandon Reauthorize

Explicitly abandon a staged re-authorization session (spec 00b §2.5).

Retires ONLY the staged pending key and clears meta.pending_reauth; the live authorization — status, signer, expiry, execution path — is untouched, exactly as if the reauth had never been started. Idempotent: with no staged session this is a no-op 204, so duplicate abandon calls converge.

The "only the pending key" guard is real, not assumed: the staged key's state is read from the signing service first. A staged key that already ACTIVATED (a partially-completed reauth whose venue-side swap succeeded) is refused with 409 — abandoning at that point would retire the signer the venue now recognizes; the recovery is to finish the swap by re-running authorization.