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/expected/0030-basic-builtins.stdout

@@ -2,12 +2,12 @@
 out-ok
 sqrt: 3.000000
 sqrt-f64: 4.000000
-sizeof-s32: 4
+sizeof-i32: 4
 sizeof-f64: 8
 sizeof-struct: 8
 alignof-u8: 1
-alignof-s32: 4
-alignof-s64: 8
+alignof-i32: 4
+alignof-i64: 8
 alignof-struct: 4
 typeof: int
 typeof-float: float