Set Clip Banner Text

Set (or clear) the TEXTUAL banner on an ALREADY-FINISHED clip (v2 §4.6): a hook address (lane and rank, resolved server-side), a custom text of at most 400 characters, or clear set to true. This is a text-only intent with no render, no burn, and no finishing quota spent. Provide exactly one of the three shapes. Recover from scope_missing by minting a finish:write key, run_not_ready by polling progress until the run completes, run_not_found or clip_not_found by listing your runs and their clips, and validation_error by providing exactly one valid shape (a real lane and rank, a non-empty text, or clear). For service_unavailable, wait retry_after_sec and retry once.