lang: rename signed integer types sN -> iN

Surface rename of the signed integer family: s1..s64 become i1..i64
(u1..u64, usize, isize unchanged). 'string' keeps the s-prefix arm in
name classification; width parsing moves to the i-prefix arm next to
isize.

Internal TypeId tags follow the surface (.s8/.s16/.s32/.s64 ->
.i8/.i16/.i32/.i64), as do mono-key mangle fragments (ptr_i64,
tu_i64_bool) and all display/diagnostic formatting (i{d}).

Migrated in the same sweep: stdlib + examples + issue repros + FFI C
companions (shared symbol names like ffi_id_i64), expected
stdout/stderr/ir snapshots, specs.md, readme.md, CLAUDE.md/AGENTS.md,
implementation_plan.md, docs/, issue writeups. Vendored stb_image and
historical flow state left untouched.

zig build test: 426/426; examples suite: 595/595.

examples/0721-modules-qualified-terminating-callee.sx

@@ -20,7 +20,7 @@ report :: (label: string, ok: bool) {
     if ok { print("{}: ok\n", label); } else { print("{}: FAIL\n", label); }
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     // Qualified callee whose body terminates via a constant-folded `if true`.
     x := m.foo();
     // Caller statements AFTER the call must still be emitted (not dead).