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/arith.sx

@@ -4,7 +4,7 @@
 #import "modules/std.sx";
 t :: #import "test.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     t.expect(2 + 2 == 4, "two plus two is four");
     t.expect(7 % 3 == 1, "seven mod three is one");
     t.expect(10 - 4 == 6, "ten minus four is six");