Attach an existing project to an enrolled instance

Attaches an existing standalone project to an enrolled standalone instance and assigns the project's region and cname from the stamp whose gateway terminates that instance's control stream. The optional location field is reserved for multi-region use and is ignored today. This operation is dual-authorized. A user-derived bearer plus the X-Org-Name assertion authorizes the project mutation; OAuth client credentials are refused. The caller must have the instances:write and apps:write scopes and the organization admin or owner role. The cert_pem and pop_sig body fields prove only that the request concerns the named instance. Instance proof by itself never authorizes project attachment. pop_sig is a base64 DER ECDSA (SHA-256) signature over the UTF-8 bytes spice-cloud-connect/attach/v1\n{X-Org-Name}\n{instance_id}\n{project_id}. The project must already have kind standalone and no cluster target. The database is the authority for attachment conflicts; clients must not pre-check or infer an attachment limit from this contract. Errors use {code, error, retryable}. A 409 always has retryable: false; a 5xx has retryable: true so the client exact replays any request whose attachment may already have committed.