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.

issues/0127-namespaced-generic-call-result-unbound-stub.md

@@ -7,13 +7,13 @@
 > the bare-identifier path routes generics through
 > `inferGenericReturnType`. Lowering dispatched the right mono (the
 > value was correct); only the planned result type was wrong, so
-> pack-fn callers (print's Any boxing) mis-tagged it — and a non-s64
+> pack-fn callers (print's Any boxing) mis-tagged it — and a non-i64
 > binding (f64) failed LLVM verification outright, the pack being
 > monomorphized for the stub while the call returned `double`. Fix:
 > both arms now classify a `type_params.len > 0` callee as
 > `.generic_fn` and infer the return type through the call's bindings,
 > mirroring the flat path. Regression test:
-> `examples/0213-generics-namespaced-call-result.sx` (s64 + f64
+> `examples/0213-generics-namespaced-call-result.sx` (i64 + f64
 > bindings via print, concrete type flowing into arithmetic; pre-fix:
 > `T{}` boxing / LLVM verification failure — both demonstrated).
 > Gates: zig build test 426/426, suite 595/595, distribution repo