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/1417-ffi-jni-class-07-all-runtimes.sx

@@ -18,11 +18,11 @@
 #import "modules/std.sx";
 
 IFoo :: #foreign #jni_interface("com/example/IFoo") {
-    bar :: (self: *Self) -> s32;
+    bar :: (self: *Self) -> i32;
 }
 
 NSString :: #foreign #objc_class("NSString") {
-    length :: (self: *Self) -> s32;
+    length :: (self: *Self) -> i32;
 }
 
 NSCopying :: #foreign #objc_protocol("NSCopying") {
@@ -38,10 +38,10 @@ Date :: #foreign #swift_struct("Foundation.Date") {
 }
 
 Hashable :: #foreign #swift_protocol("Swift.Hashable") {
-    hash :: (self: *Self) -> s32;
+    hash :: (self: *Self) -> i32;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("parse-only ok\n");
     0
 }