I may have been premature in saying I was done with Braid. I may also have been premature in saying it was a game about relationships. Apparently there is more to the game than just assembling all the puzzles and seeing the final level. Which is, I think, a shame. Sometimes games need to know when to keep their mouth shut.
Okay, so in the hub world there's a screen with a constellation, right? Apparently there's eight stars hidden in the game, and finding them all reveals the image of a chained princess in the sky in the hub world. Also, finding the last star involves doing something in the last level which... I don't know how to finish that sentence without spoilers, but as far as I'm concerned it ruins that level. And the rest of the game.
And then it turns out the whole game wasn't about relationships after all but was instead about the nuclear bomb.
Bollocks. Bollocks, I say. I am going to promptly forget I ever discovered this and I suggest you do too.
If you want to know more there's an FAQ here but you should be aware that:
(a) getting the stars is stupidly hard
(b) you can't do it if you've already finished the game - you'll have to start from scratch, and
(c) getting a single star can take up to two hours, and there are eight of them.
Thanks to Phrancq for the tip-off.