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/1008-errors-match-diverging-arms.sx

@@ -9,7 +9,7 @@
 #import "modules/std.sx";
 
 // All arms diverge — the match is `noreturn`, no merge phi.
-classify :: (n: s32) -> s32 {
+classify :: (n: i32) -> i32 {
     if n == {
         case 0: return 10;
         case 1: return 20;
@@ -19,7 +19,7 @@ classify :: (n: s32) -> s32 {
 }
 
 // Mixed: value arms + a diverging arm.
-pick :: (n: s32) -> s32 {
+pick :: (n: i32) -> i32 {
     v := if n == {
         case 0: 1;
         case 1: return 100;         // diverging arm — no fallback const after its `ret`
@@ -28,8 +28,8 @@ pick :: (n: s32) -> s32 {
     return v + 5;
 }
 
-main :: () -> s32 {
-    r : s32 = 0;
+main :: () -> i32 {
+    r : i32 = 0;
     r = r + classify(0);            // 10
     r = r + classify(1);            // 20
     r = r + classify(7);            // 90