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/0301-closures-fn-pointers.sx

@@ -1,15 +1,15 @@
 #import "modules/std.sx";
 
-add :: (a: s32, b: s32) -> s32 { a + b }
-mul :: (a: s32, b: s32) -> s32 { a * b }
+add :: (a: i32, b: i32) -> i32 { a + b }
+mul :: (a: i32, b: i32) -> i32 { a * b }
 
-apply :: (f: (s32, s32) -> s32, x: s32, y: s32) -> s32 {
+apply :: (f: (i32, i32) -> i32, x: i32, y: i32) -> i32 {
     f(x, y)
 }
 
 main :: () {
     // Store function in variable
-    fp : (s32, s32) -> s32 = add;
+    fp : (i32, i32) -> i32 = add;
     print("fp(3,4) = {}\n", fp(3, 4));
 
     // Reassign to different function