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 on the response reflects the row's role — the side that initiated the cancel.

The provider call is synchronous against the simulator; on success the response returns status = cancelled. 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.