A second, fixed-menu artifact fast-acknowledgment selector exists behind two gates that both default off

Alongside the existing free-form fast-acknowledgment composer for artifact comment threads, a new selector mode asks the model to pick one numbered acknowledgment from a fixed list, instructed to output only its number. It runs with a 5-token output cap, a deadline, and its own failure telemetry (fast_ack_select_error, fast_ack_select_unparsed, fast_ack_select_timeout, fast_ack_select_threw). It also adds a permission shortcut that can reuse a prior probe verdict instead of re-asking.

This selector only runs when CLAUDE_CODE_ARTIFACT_COMMENT_FAST_ACK_FIXED (or its underlying gate) is true, and that check is nested inside the pre-existing fast-ack gate that governs the original composer. Both default to false, so turning this on requires opting in to both.