migrate to the new for-loop syntax

Drop the ':' before captures (for xs (x) / for 0..n (i)); the index
capture becomes the trailing open range (for xs, 0.. (x, i)). 136
headers across 26 files, mechanical.

Five headless tests (banner_layout, hit_test, swipe_commit,
swipe_intent, swipe_reshuffle) also gain a direct
#import "modules/ui/types.sx" — they named Point/Frame through a
transitive import, which bare visibility no longer permits.

Gates: sx build --target ios-sim main.sx links; tools/run_tests.sh
23/23.

tests/swipe_intent.sx

@@ -9,6 +9,7 @@
 // clears it; a 10px drag does not. Failure is signalled via a non-zero exit code
 // (the runner checks exit code AND stdout).
 #import "modules/std.sx";
+#import "modules/ui/types.sx";
 #import "board.sx";
 #import "board_layout.sx";
 #import "swipe.sx";