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/easing.sx

@@ -19,8 +19,8 @@
 fabs :: (x: f32) -> f32 { if x < 0.0 then 0.0 - x else x }
 approx :: (a: f32, b: f32) -> bool { fabs(a - b) < 0.0001 }
 
-main :: () -> s32 {
-    fails : s64 = 0;
+main :: () -> i32 {
+    fails : i64 = 0;
 
     // 1. Endpoints are locked: every curve starts/ends exactly on its rest value
     //    (the in/out curves at 1, the spring at 1, the squash envelope at 0).