Cancel a proxy claim. Allowed start status depends on the claim type:

• type=portability — cancellable from open or waiting_resolution.
• type=ownership — cancellable from open, waiting_resolution, or confirmed.

Either role (claimer or donor) can call cancel. The
scheme_data.cancelled_by field reflects the row's role — the side
that initiated the cancel — and is stamped on this call.

Cancellation is async at the scheme. On success the response returns
status = pending_cancellation: the request was accepted and is awaiting
the scheme to finalise it. The claim is flipped to cancelled by the
proxy_claim.cancellation_completed inbound event. On provider rejection
the response returns 200 with status = cancellation_rejected and a
populated status_details (rejections are stored business states, not
HTTP errors).

Cancellation does not affect the linked proxy_key: if an ownership
claim is cancelled after being confirmed (e.g. DICT FRAUD), the
donor-side proxy_key.disabled state is not auto-reverted. Manual
reconciliation if needed.