50-move rule: only the last 50 or any consecutive 50? Suppose that, as a result of White's move, there have now been 100 single-moves (50 full moves) since the most recent pawn move or capture. White didn't claim a draw because they're going for the win. Every option for Black's next move is a pawn move or a capture. Does this mean that Black has now lost the opportunity to claim a draw under the 50-move rule?

(Art. 9.3 of FIDE's Laws of Chess (2018) suggests that they have, but I wanted to check. If I've understood correctly, the opportunity does not just occur on the 100th non-pawn non-capture and then vanish forever --- you may claim at any point from the 100th onwards in a series of more than 100 --- but there must be a series of at least 100 including the very turn in which you make your claim. That is, retrospective claims do not stand.) 

This question was prompted by my reading up on Art. 9.3 of FIDE's Laws of Chess (2018) to respond to this question.
She can claim a draw. Article 9.3 of the FIDE Laws of Chess, 2018, emphasis mine:

9.3 The game is drawn, upon a correct claim by a player having the move, if:
9.3.1 he writes his move, which cannot be changed, on his scoresheet and declares to the arbiter his intention to make this move which will result in thelast 50 moves by eachplayer having been made without the movement of any pawn and without any capture, or
9.3.2 the last 50 moves by each player have been completed without the movement of any pawn and without any capture.

9.3.2 is the case you describe, where the moves have already been completed. 9.3.1 is the case that counts the move the player is about to make.
Edit: but you link to the other question. In real tournament chess, checkmate would certainly trump this -- checkmate ends the game, so the other player would never get another turn to claim a draw in. But problems are not tournament chess and they use their own conventions, like the famous rule that castling is allowed unless you can prove it isn't.