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.

library/modules/ffi/objc.sx

@@ -26,8 +26,8 @@ SEL   :: *void;
 
 // Apple's `BOOL` is a signed char (NOT sx's built-in `bool`, which is
 // LLVM `i1`). Obj-C method signatures that take or return `BOOL` cross
-// the FFI boundary as `s8`.
-BOOL :: s8;
+// the FFI boundary as `i8`.
+BOOL :: i8;
 
 // On macOS libobjc is auto-loaded by libSystem; on iOS it isn't, so we
 // link it explicitly. Foundation registers NSString etc. with the runtime,