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/1341-ffi-objc-dsl-01-niladic.sx

@@ -17,14 +17,14 @@
 #import "modules/ffi/objc.sx";
 
 SxProbeNiladic :: #foreign #objc_class("SxProbeNiladic") {
-    length :: (self: *Self) -> s32;
+    length :: (self: *Self) -> i32;
 }
 
-length_imp :: (self: *void, _cmd: *void) -> s32 callconv(.c) {
+length_imp :: (self: *void, _cmd: *void) -> i32 callconv(.c) {
     42
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     inline if OS == .macos {
         ns_object := objc_getClass("NSObject".ptr);
         cls := objc_allocateClassPair(ns_object, "SxProbeNiladic".ptr, 0);