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/clear.sx

@@ -38,7 +38,7 @@ load_board :: (rows: []string) -> Board {
 // Detect→clear one scene, snapshot before/after, and assert the three clear
 // invariants against the matched-cell set: every flagged cell is now a hole,
 // every unflagged cell is unchanged, and the returned count is exact.
-scene :: (name: string, rows: []string, want_cleared: s64) {
+scene :: (name: string, rows: []string, want_cleared: i64) {
     b := load_board(rows);
     orig := load_board(rows);          // pristine copy for the unchanged check
 
@@ -65,7 +65,7 @@ scene :: (name: string, rows: []string, want_cleared: s64) {
     t.expect(cleared == want_cleared, concat(name, ": cleared count exact"));
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("== clear (detect -> clear) ==\n");
 
     // Single horizontal 3-run (row 3, cols 2-4) → three holes there only.