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.

issues/0072-global-field-access-const-initializer-silent-zero.md

@@ -35,14 +35,14 @@ initializer loudly if field-access global constants are not supported yet.
 #import "modules/std.sx";
 
 Point :: struct {
-    x: s32;
-    y: s32;
+    x: i32;
+    y: i32;
 }
 
 K : Point : Point.{ x = 9, y = 4 };
-g : s32 = K.x;
+g : i32 = K.x;
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("g={}\n", g);
     return g;
 }
@@ -68,7 +68,7 @@ Context:
   (`K : A : 42; g : A = K;`) and diagnoses identifiers that are not usable
   constants.
 - A remaining non-identifier expression shape still falls through silently:
-  `K : Point : Point.{ x = 9, y = 4 }; g : s32 = K.x;` emits a null global
+  `K : Point : Point.{ x = 9, y = 4 }; g : i32 = K.x;` emits a null global
   initializer payload and runs as `g=0`.
 
 Suspected area: