Chat: a send stamped before the first transcript read can be settled by an earlier identical turn in the same second #3166

Follow-up from the review of #3165 (send stamp on submit-result, fixes #2974) at head dc5c66e.

A stamped send made before the chat's first transcript read skips the whole-second ambiguity guard (src/lib/sentWaiting.ts:188).

Repro (reproduced against the client logic with a recorded transcript): a Grok transcript records an earlier repeat and its answer at 10:00:02 (whole-second times). The browser sends another repeat at 10:00:02.700 and gets the PC's sending_at stamp. When the earlier transcript arrives before the new turn is written, the client clears the new waiting copy as if that earlier turn confirmed it. With the unstamped baseline the waiting copy stays; with the stamped baseline it is dropped.

Expected: for transcripts with whole-second times, a turn in the same second as the stamp only settles the waiting copy when it is newer than the turns already known (the guard the unstamped path keeps), including when the stamp comes before the first transcript read.