Tap a card's button

Dispatch is on the button's own server-recorded action_type, NEVER its label: pay creates a real TransferRequest against the owner's matching-currency wallet (refused for a guest session -- see 403); handoff runs the same authorized hand-off chats#hand_off uses; signup (guest chats) is never forwarded anywhere; anything else forwards to the card owner's webhook/socket outbox as a card_interaction event. values is an optional {block_id => answer} map answering the card's own input blocks -- every key must name an input block this card declares and every value must be a string no longer than that block's own max_length, else a 422 with one plain sentence. A pay, handoff or signup button ignores values entirely.