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/0753-modules-same-name-struct-same-fields/a.sx

@@ -1,3 +1,3 @@
 // Module A authors `Pair { x, y }`.
-Pair :: struct { x: s64; y: s64; }
+Pair :: struct { x: i64; y: i64; }
 a_pair :: () -> Pair { return Pair.{ x = 1, y = 2 }; }

examples/0753-modules-same-name-struct-same-fields/b.sx

@@ -1,5 +1,5 @@
 // Module B authors `Pair { x, y }` with the SAME field shape as A's. The two
 // still get distinct nominal identities (not collapsed): each keeps its own
 // TypeId / per-source author, so both register and format independently.
-Pair :: struct { x: s64; y: s64; }
+Pair :: struct { x: i64; y: i64; }
 b_pair :: () -> Pair { return Pair.{ x = 5, y = 6 }; }