RPC extension UI

RPC mode binds a functional extension UI transport. A dialog emits an extension_ui_request record and waits for the matching extension_ui_response.

Example selector request:

{"type":"extension_ui_request","id":"...","method":"select","title":"Choose","options":["A","B"]}

Return a value, confirmation, or cancellation:

{"type":"extension_ui_response","id":"...","value":"A"}

{"type":"extension_ui_response","id":"...","confirmed":true}

{"type":"extension_ui_response","id":"...","cancelled":true}

PiG supports select, confirm, input, and editor as blocking dialogs. It emits fire-and-forget requests for notify, setStatus, setWidget, setTitle, and set_editor_text. TUI-only component factories remain no-ops in RPC mode.