Signature
TimelineCommandFailureReason: "not-found" | "locked" | "invalid-range" | "invalid-duration" | "invalid-track" | "incompatible-track-kind" | "duplicate-id" | "disabled" | "content-gap" | "empty-selection" | "empty-clipboard" | "out-of-bounds" | "policy-rejected" | "source-bounds" | "unsupported"