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/match_detect.sx

@@ -20,7 +20,7 @@ char_to_gem :: (c: u8) -> Gem {
 // Build a scene: load an 8x8 board from `rows` (top row first, each exactly
 // BOARD_COLS gem characters), detect matches, print board + matched dump, and
 // assert the matched-cell count.
-scene :: (name: string, rows: []string, want_count: s64) {
+scene :: (name: string, rows: []string, want_count: i64) {
     b : Board = ---;
     for 0..BOARD_ROWS (row) {
         line := rows[row];
@@ -38,7 +38,7 @@ scene :: (name: string, rows: []string, want_count: s64) {
     t.expect(m.count() == want_count, name);
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     // Single horizontal 3-run (row 3, cols 2-4).
     scene("horizontal-3", .[
         "OGOGOGOG",