Authorization rule

This is the Canton ledger model's rule, enforced by K with the stakeholder sets as witness:

1. Actors are the parties whose Schnorr signatures over manifest_hash the fragment carries (up to 4).
2. Exercise. The method circuit asserts that its template's declared controllers for this choice are a subset of the actors. K verifies the signatures and the actors' admission (§4.5).
3. Create. For every created record, every signatory is either an actor or a signatory of a consumed record in this fragment. Signatories of a consumed record delegate authority to whoever exercises its choice, which is what makes propose-and-accept, delegation, and settlement choices work without both parties being present.
4. Observers grant no authority and require none.

Weld's hold is the smallest instance: the sender is the sole signatory of the proposal, the recipient FI's customer exercises accept, and the created recipient note is authorized by the proposal's signatory.