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/1016-errors-onfail.sx

@@ -9,13 +9,13 @@
 
 E :: error { Bad }
 
-inner :: (n: s32) -> !E {
+inner :: (n: i32) -> !E {
     if n < 0 { raise error.Bad; }
     return;
 }
 
 // defer + onfail interleave on the error path; only defers on success.
-run :: (n: s32) -> !E {
+run :: (n: i32) -> !E {
     defer  print("defer A\n");
     onfail print("onfail B\n");
     defer  print("defer C\n");
@@ -24,13 +24,13 @@ run :: (n: s32) -> !E {
 }
 
 // `onfail e` binds the tag.
-classify :: (n: s32) -> !E {
+classify :: (n: i32) -> !E {
     onfail (e) { if e == error.Bad { print("cleanup: bad\n"); } }
     if n < 0 { raise error.Bad; }
     return;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("[fail]\n");
     a := run(-1);                       // error → defer C, onfail B, defer A
     print("[ok]\n");