I've rounded the windward mark and borne away — doesn't the boat still beating have to keep clear?
No. Rule 10 is about tack, not direction: on opposite tacks the port-tack boat keeps clear of the starboard-tack boat whether the boats are going upwind, downwind or across. Bearing away after the mark did not change your tack, so on port you still owe the duty. The starboard boat's late dodge is clean under Rule 14 — a right-of-way boat need not act until it is clear the other boat is not keeping clear.
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