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/1320-ffi-objc-extends-chain.sx

@@ -23,7 +23,7 @@ NSObjectBase :: #foreign #objc_class("NSObject") {
 // walk through NSObjectBase, then dispatched by objc_msgSend.
 SxThing :: #objc_class("SxThing") {
     #extends NSObjectBase;
-    counter: s32;
+    counter: i32;
 
     alloc :: () -> *SxThing;
     init  :: (self: *SxThing) -> *SxThing;
@@ -41,7 +41,7 @@ SxLeaf :: #objc_class("SxLeaf") {
     init  :: (self: *SxLeaf) -> *SxLeaf;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     inline if OS == .macos {
         // 1-level chain: SxThing → NSObjectBase.
         t := SxThing.alloc().init();