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/1048-errors-cleanup-absorption.sx

@@ -9,9 +9,9 @@
 E :: error { Bad }
 
 failing :: () -> !E { raise error.Bad; }
-recover :: () -> (s32, !E) { raise error.Bad; }
+recover :: () -> (i32, !E) { raise error.Bad; }
 
-work :: (n: s32) -> !E {
+work :: (n: i32) -> !E {
     defer  print("defer: always\n");                                 // plain cleanup
     onfail { failing() catch (e) print("onfail: caught (catch)\n"); }  // catch absorbs
     onfail { x := recover() or 7; print("onfail: x={} (or)\n", x); } // or-value absorbs
@@ -19,7 +19,7 @@ work :: (n: s32) -> !E {
     return;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("[error]\n");
     a := work(-1);   // raises → onfail bodies fire, then defer (reverse decl order)
     print("[ok]\n");