lang migration: rename signed integer types sN -> iN

Mechanical sweep of all .sx sources and plan docs (PLAN.md, current/,
.agents/) for the sx language rename (s8/s16/s32/s64 -> i8/i16/i32/i64).
Verified: make build + make test, 14/14.

tests/cli_dispatch.sx

@@ -45,7 +45,7 @@ contains :: (hay: string, needle: string) -> bool {
     return false;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     gpa := GPA.init();
 
     // ── 1. No command → readable usage on stderr, EX_USAGE (64) ───────