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/level.sx

@@ -34,7 +34,7 @@ char_to_gem :: (c: u8) -> Gem {
 // Load an 8x8 board from `rows` (top row first, each exactly BOARD_COLS chars),
 // seeded RNG, running score zeroed, the turn counters reset to a fresh game, and
 // the per-level goal set.
-load_board :: (rows: []string, seed: s64, move_limit: s64, target_score: s64) -> Board {
+load_board :: (rows: []string, seed: i64, move_limit: i64, target_score: i64) -> Board {
     b : Board = ---;
     for 0..BOARD_ROWS (row) {
         line := rows[row];
@@ -55,7 +55,7 @@ boards_equal :: (a: *Board, b: *Board) -> bool {
     true
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("== level (turn / goal state machine) ==\n");
 
     // ── Start: a fresh seeded board reads in_progress with the default goal ──