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/1012-errors-value-failable-consume.sx

@@ -10,32 +10,32 @@
 
 E :: error { Bad, Empty }
 
-parse :: (n: s32) -> (s32, !E) {
+parse :: (n: i32) -> (i32, !E) {
     if n < 0  { raise error.Bad; }
     if n == 0 { raise error.Empty; }
     return n * 2;
 }
 
 // value-carrying `try` in a value-carrying caller — propagates {undef, tag}.
-inc :: (n: s32) -> (s32, !E) {
+inc :: (n: i32) -> (i32, !E) {
     v := try parse(n);
     return v + 1;
 }
 
 // value-carrying `try` in a pure-failable caller — propagates the tag.
-relay :: (n: s32) -> !E {
+relay :: (n: i32) -> !E {
     v := try parse(n);
     if v < 0 { raise error.Bad; }
     return;
 }
 
 // value-carrying `catch`, bare-expression fallback.
-safe :: (n: s32) -> s32 {
+safe :: (n: i32) -> i32 {
     return parse(n) catch (e) 0;
 }
 
 // value-carrying `catch`, match-body value.
-classify :: (n: s32) -> s32 {
+classify :: (n: i32) -> i32 {
     return parse(n) catch (e) == {
         case .Bad:   1;
         case .Empty: 2;
@@ -43,8 +43,8 @@ classify :: (n: s32) -> s32 {
     };
 }
 
-main :: () -> s32 {
-    r : s32 = 0;
+main :: () -> i32 {
+    r : i32 = 0;
     a, ea := inc(5);                      // parse(5)=10 → v=10 → 11
     if !ea { r = r + a; }                  // success → +11 (value live only when proven ok)
     b, eb := inc(-1);                      // parse(-1)=Bad → propagate {undef, Bad}