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/1009-errors-catch.sx

@@ -12,14 +12,14 @@
 
 E :: error { Bad, Empty }
 
-must :: (n: s32) -> !E {
+must :: (n: i32) -> !E {
     if n < 0  { raise error.Bad; }
     if n == 0 { raise error.Empty; }
     return;
 }
 
 // Diverging body — returns from `classify` on error.
-classify :: (n: s32) -> s32 {
+classify :: (n: i32) -> i32 {
     must(n) catch (e) {
         if e == error.Bad   { return 1; }
         if e == error.Empty { return 2; }
@@ -29,7 +29,7 @@ classify :: (n: s32) -> s32 {
 }
 
 // Match-body form — sugar for `catch (e) { if e == { case ... } }`.
-mclassify :: (n: s32) -> s32 {
+mclassify :: (n: i32) -> i32 {
     must(n) catch (e) == {
         case .Bad:   return 11;
         case .Empty: return 22;
@@ -40,7 +40,7 @@ mclassify :: (n: s32) -> s32 {
 
 // Selective handle + re-raise (failable enclosing fn; `raise e` is the
 // variable form). Swallows Bad → success; re-raises everything else.
-handle_some :: (n: s32) -> !E {
+handle_some :: (n: i32) -> !E {
     must(n) catch (e) {
         if e == error.Bad { return; }       // swallow → success
         raise e;                            // re-raise the rest
@@ -48,8 +48,8 @@ handle_some :: (n: s32) -> !E {
     return;
 }
 
-main :: () -> s32 {
-    r : s32 = 0;
+main :: () -> i32 {
+    r : i32 = 0;
     must(-1) catch (e) { if e == error.Bad { r = r + 1; } };  // Bad → +1
     must(5)  catch { r = r + 100; };                         // success → body skipped
     r = r + classify(0);                                     // Empty → 2