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/board_init.sx

@@ -10,8 +10,8 @@ SEED :: 1337;
 // Count every horizontal or vertical window of three consecutive same-type
 // gems. A correctly initialized board has zero. This walks the finished board
 // independently of the placement logic, so it's a real check, not a tautology.
-count_three_runs :: (b: *Board) -> s32 {
-    runs : s32 = 0;
+count_three_runs :: (b: *Board) -> i32 {
+    runs : i32 = 0;
     for 0..BOARD_ROWS (row) {
         for 0..(BOARD_COLS - 2) (col) {
             g := b.at(col, row);
@@ -27,7 +27,7 @@ count_three_runs :: (b: *Board) -> s32 {
     runs
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     board : Board = ---;
     board.init(SEED);