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/server_http.sx

@@ -100,7 +100,7 @@ publish_cmd :: (mpath: string) -> string {
     return concat(c, " --json 2>/dev/null >/dev/null");
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     gpa := GPA.init();
     arena := Arena.init(xx gpa, 1 << 20);
     defer arena.deinit();