Attach an antigen target to the candidate.
Both the candidate and the antigen must walk back to the caller’s
org; either-side cross-org hits return 404. Links are separate
from the antigen_id supplied at complex-prediction submit time
— this endpoint maintains the cataloguing relationship; complex
predictions are free to target any antigen in the caller’s org
regardless of the candidate’s linked set.
Cost: write — burns rate + quota. Does not debit credits.