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/1312-ffi-objc-class-method-dispatch.sx

@@ -18,18 +18,18 @@
 class_getMethodImplementation :: (cls: *void, sel: *void) -> *void #foreign objc;
 
 SxFoo :: #objc_class("SxFoo") {
-    counter: s32;
+    counter: i32;
 
     bump :: (self: *Self) {
         self.counter += 1;
     }
 
-    add :: (self: *Self, n: s32) {
+    add :: (self: *Self, n: i32) {
         self.counter += n;
     }
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     inline if OS == .macos {
         cls : Class = objc_getClass("SxFoo".ptr);
         if cls == null { print("FAIL: SxFoo not registered\n"); return 1; }