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/0405-protocols-enum-through-protocol-dispatch.sx

@@ -12,17 +12,17 @@ Proto :: protocol {
 Impl :: struct {}
 impl Proto for Impl {
   take_fmt :: (self: *Impl, f: Fmt) {
-    n : s64 = xx f;
+    n : i64 = xx f;
     print("proto f = {}\n", n);
   }
 }
 
-take :: (f: Fmt) -> s64 {
-  n : s64 = xx f;
+take :: (f: Fmt) -> i64 {
+  n : i64 = xx f;
   n
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
   print("direct a={} b={}\n", take(.a), take(.b));
 
   p : Proto = xx @Impl.{};