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/0530-packs-pack-interface-only.sx

@@ -9,14 +9,14 @@
 Box :: protocol(T: Type) {
     get :: () -> T;
 }
-IntCell :: struct { v: s64; }
-impl Box(s64) for IntCell { get :: (self: *IntCell) -> s64 => self.v; }
+IntCell :: struct { v: i64; }
+impl Box(i64) for IntCell { get :: (self: *IntCell) -> i64 => self.v; }
 
-leak :: (..xs: Box) -> s64 {
+leak :: (..xs: Box) -> i64 {
     return xs[0].v;   // `v` is not part of Box — error
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("{}\n", leak(IntCell.{ v = 5 }));
     0
 }