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/0752-modules-same-name-struct-distinct-fields/a.sx

@@ -1,3 +1,3 @@
-// Module A authors its OWN `Box` (one `s64` field `x`).
-Box :: struct { x: s64; }
+// Module A authors its OWN `Box` (one `i64` field `x`).
+Box :: struct { x: i64; }
 a_box :: () -> Box { return Box.{ x = 7 }; }

examples/0752-modules-same-name-struct-distinct-fields/b.sx

@@ -1,5 +1,5 @@
 // Module B authors a DIFFERENT `Box` (two fields `p`, `q`) — a same-name shadow
 // of A's `Box`. Pre-0105 the two collapsed last-wins in the type table, so one
 // module's field set vanished; now each `Box` is a distinct nominal type.
-Box :: struct { p: s64; q: s64; }
+Box :: struct { p: i64; q: i64; }
 b_box :: () -> Box { return Box.{ p = 3, q = 4 }; }