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/1142-diagnostics-reserved-name-struct-const.sx

@@ -1,6 +1,6 @@
 // A bare reserved/builtin type-name spelling is rejected as the NAME of a
-// STRUCT-BODY constant too — both the untyped (`s2 :: 5`) and the typed
-// (`u8 : s64 : 9`) forms — exactly like a top-level const (examples/1140) or a
+// STRUCT-BODY constant too — both the untyped (`i2 :: 5`) and the typed
+// (`u8 : i64 : 9`) forms — exactly like a top-level const (examples/1140) or a
 // type decl (examples/1141). A struct member constant is a binding site, so a
 // bare reserved spelling mis-classifies and is rejected; the caret lands ON the
 // constant's name (not at 1:1). The backtick escape (examples/0156) is the only
@@ -11,10 +11,10 @@
 #import "modules/std.sx";
 
 Holder :: struct {
-    s2 :: 5;
-    u8 : s64 : 9;
+    i2 :: 5;
+    u8 : i64 : 9;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     return 0;
 }