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/1018-errors-multi-value-failable.sx

@@ -12,26 +12,26 @@
 
 E :: error { Bad, Empty }
 
-parse :: (n: s32) -> (s32, s32, !E) {
+parse :: (n: i32) -> (i32, i32, !E) {
     if n < 0  { raise error.Bad; }
     if n == 0 { raise error.Empty; }
     return (n * 2, n + 1);            // success → {n*2, n+1, 0}
 }
 
 // Multi-value `try` in a multi-value caller — propagates {undef, undef, tag}.
-inc :: (n: s32) -> (s32, s32, !E) {
+inc :: (n: i32) -> (i32, i32, !E) {
     v, b := try parse(n);
     return (v + 1, b + 1);
 }
 
 // Multi-value `catch`, bare-expression tuple fallback (absorbs the failure).
-safe :: (n: s32) -> s32 {
+safe :: (n: i32) -> i32 {
     v, b := parse(n) catch (e) (40, 50);
     return v + b;
 }
 
 // Multi-value `catch` match-body — per-tag dispatch, each arm a value-tuple.
-classify :: (n: s32) -> s32 {
+classify :: (n: i32) -> i32 {
     v, b := parse(n) catch (e) == {
         case .Bad:   (1, 1);
         case .Empty: (2, 2);
@@ -41,13 +41,13 @@ classify :: (n: s32) -> s32 {
 }
 
 // Multi-value `or (tuple)` value-terminator (absorbs the failure).
-ortest :: (n: s32) -> s32 {
+ortest :: (n: i32) -> i32 {
     v, b := parse(n) or (7, 8);
     return v + b;
 }
 
-main :: () -> s32 {
-    r : s32 = 0;
+main :: () -> i32 {
+    r : i32 = 0;
 
     // Destructure binds EVERY slot including the error tag (e1 / e2 / e3) —
     // the error is treated, never dropped.