I had mark-room and tacked to round — so I was still protected, right?
No. The instant your bow passes head to wind, Rule 18 switches off (Rule 18.2(a)(2)) — your mark-room is gone and you're back under the basic right-of-way rules. Now you're on port, she's on starboard, and you must keep clear under Rule 10; while still tacking you also owe Rule 13. Rule 15 doesn't help you — you acquired right of way by your own tack. Fail to keep clear and the contact is your foul under Rule 14. (World Sailing Case 81.)
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