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/refill.sx

@@ -41,8 +41,8 @@ load_board :: (rows: []string) -> Board {
     b
 }
 
-count_empties :: (b: *Board) -> s64 {
-    n : s64 = 0;
+count_empties :: (b: *Board) -> i64 {
+    n : i64 = 0;
     for 0..BOARD_CELLS (i) { if b.cells[i] == .empty { n += 1; } }
     n
 }
@@ -72,7 +72,7 @@ fresh_board :: () -> Board {
     b
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("== refill (seeded) ==\n");
 
     // Pipeline, snapshotting each stage.
@@ -123,7 +123,7 @@ main :: () -> s32 {
     // filled, then refill again. The board's RNG has advanced past the first
     // fill, so the second fill draws new gems — proof it does NOT reseed per call.
     holes_n := 0;
-    hole_idx : [BOARD_CELLS]s64 = ---;
+    hole_idx : [BOARD_CELLS]i64 = ---;
     fill1 : [BOARD_CELLS]Gem = ---;
     for 0..BOARD_CELLS (i) {
         if pre.cells[i] == .empty {