I thought I had lost, but Cosumi says that I (white) won the game (I think by 27 points).
I don't see how those 7 black stones are prisoners rather than the adjacent 4 white stones.
Am I missing something?
This game isn't really finished - there are still moves available that gain points.
If it's your turn, playing in the center and then extending upward if Black connects will capture at least the five stones that are transparent. Once you've done that, the four white stones to the right will be safe, and you can capture two more black stones by sacrificing one along the edge.
This is an intriguing position. TimK already explained that a white move at F6 will capture 5 black stones and that this is not because black cannot save them, but because saving them would lead to desaster.
To illustrate his answer, at first, it looks like white cannot do anything because the stones at H11 are in atari. However, if white forces black into a dumpling shape and to capture them, white can then cut at C7 and black is in trouble.
(5 at 8)
Per TimK, at this point, the black-10 group is in fact captured: Black cannot capture white-11, nor can he capture D9. This is a complete collapse.
The best endgame after white-1 might be, as TimK suggests, to simply answer white-1 at white-3, sacrificing 5 stones. Then black must next protect the cut by playing at E7. This gives white time to capture L7 via throw-in. In total, white wins by 20 points.
I do not see any other resistance for black that would achieve a better result.