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/collapse.sx

@@ -79,7 +79,7 @@ scene :: (name: string, rows: []string, want_moved: bool) {
     t.expect(moved == want_moved, concat(name, ": moved flag exact"));
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("== collapse (gravity) ==\n");
 
     // Eight independent columns, one case each (top-to-bottom):