lang migration: rename signed integer types sN -> iN

Mechanical sweep of all .sx sources, plan docs, and tests/expected
snapshots for the sx language rename (s8/s16/s32/s64 -> i8/i16/i32/i64).
Verified: tools/run_tests.sh 23/23.

Note: the ios-sim build has 2 pre-existing 'restart' dot-call errors
from the sx opt-in UFCS change (sx a47ea14) — independent of this
rename (present pre-sweep); migrated in the follow-up commit.

tests/swipe_commit.sx

@@ -20,7 +20,7 @@
 
 SEED :: 1337;
 
-cell_center :: (lay: *BoardLayout, col: s64, row: s64) -> Point {
+cell_center :: (lay: *BoardLayout, col: i64, row: i64) -> Point {
     cf := lay.cell_frame(col, row);
     Point.{ x = cf.mid_x(), y = cf.mid_y() }
 }
@@ -32,14 +32,14 @@ boards_equal :: (x: *Board, y: *Board) -> bool {
     true
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     // 800×600, no safe inset → 600px square grid, cell 75, origin (100, 0). A
     // 60px drag clears the cell*0.5 = 37.5px swipe threshold on the dominant axis.
     lay : BoardLayout = ---;
     lay.compute(Frame.make(0.0, 0.0, 800.0, 600.0), EdgeInsets.zero());
     D : f32 = 60.0;
 
-    fails : s64 = 0;
+    fails : i64 = 0;
 
     // ── ILLEGAL swipe reverts ──────────────────────────────────────────────
     // (0,0) and (1,0) are both red on the seed board, so swapping them forms no