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/1321-ffi-objc-defined-class-method-self.sx

@@ -23,7 +23,7 @@ SxIssue44Bus :: #foreign #objc_class("NSNotificationCenter") {
 }
 
 SxIssue44Foo :: #objc_class("SxIssue44Foo") {
-    counter: s32;
+    counter: i32;
     sentinel: *void;
 
     alloc :: () -> *SxIssue44Foo;
@@ -32,7 +32,7 @@ SxIssue44Foo :: #objc_class("SxIssue44Foo") {
         this.counter += 1;
     }
 
-    get :: (this: *Self) -> s32 {
+    get :: (this: *Self) -> i32 {
         return this.counter;
     }
 
@@ -69,7 +69,7 @@ capture_observer :: (p: *void) {
     g_observer = p;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     inline if OS == .macos {
         f := SxIssue44Foo.alloc();
         if f == null { print("FAIL: alloc returned null\n"); return 1; }