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/1317-ffi-objc-class-level-constant.sx

@@ -21,24 +21,24 @@ NSObject :: #foreign #objc_class("NSObject") {
 
 // Reframed as a class method internally; user writes the constant form.
 SxThing :: #objc_class("SxThing") {
-    counter: s32;
+    counter: i32;
 
     // Class-level constant.
-    answer :: s32 = 42;
+    answer :: i32 = 42;
 
     // Canonical pattern: returning a *NSObject (stand-in for Apple's
     // '+layerClass' returning *CALayer).
     seedClass :: *NSObject = NSObject.alloc().init();
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     inline if OS == .macos {
         cls : Class = objc_getClass("SxThing".ptr);
         if cls == null { print("FAIL: SxThing not registered\n"); return 1; }
 
         // [SxThing answer] → 42
         sel_answer : SEL = sel_registerName("answer".ptr);
-        msg_int : (cls: *void, sel: *void) -> s32 callconv(.c) = xx objc_msgSend;
+        msg_int : (cls: *void, sel: *void) -> i32 callconv(.c) = xx objc_msgSend;
         r := msg_int(cls, sel_answer);
         if r != 42 { print("FAIL: answer expected 42, got {}\n", r); return 1; }