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/hit_test.sx

@@ -10,6 +10,7 @@
 // second `Frame` struct that collides with the UI `Frame`. 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";
 
@@ -26,8 +27,8 @@ main :: () -> s32 {
 
     // Every cell center must map back to its own cell.
     hits : s64 = 0;
-    for 0..BOARD_ROWS: (row) {
-        for 0..BOARD_COLS: (col) {
+    for 0..BOARD_ROWS (row) {
+        for 0..BOARD_COLS (col) {
             cf := lay.cell_frame(col, row);
             center := Point.{ x = cf.mid_x(), y = cf.mid_y() };
             if h := lay.point_to_cell(center) {