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/1301-ffi-objc-class.sx

@@ -13,7 +13,7 @@
 #import "modules/std.sx";
 #import "modules/ffi/objc.sx";
 
-g_marker : s32 = 0;
+g_marker : i32 = 0;
 
 // IMP for `hello`. Must use C calling convention so `self` and `_cmd` land in
 // x0 and x1 the way the Obj-C runtime expects.
@@ -21,7 +21,7 @@ hello_imp :: (self: *void, _cmd: *void) callconv(.c) {
     g_marker = 42;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     NSObject := objc_getClass("NSObject".ptr);
     SxThing  := objc_allocateClassPair(NSObject, "SxThing".ptr, 0);
     sel_hello := sel_registerName("hello".ptr);