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/0758-modules-same-name-struct-mutual-ref/a.sx

@@ -1,3 +1,3 @@
 // Module A authors its OWN `Box` and `Node` (the FIRST same-name authors).
-Box  :: struct { x: s64; }
-Node :: struct { n: s64; }
+Box  :: struct { x: i64; }
+Node :: struct { n: i64; }

examples/0758-modules-same-name-struct-mutual-ref/b.sx

@@ -2,9 +2,9 @@
 // B's `Box` has a FORWARD ref to B's `Node` (declared after it), and B's `Node`
 // back-refs B's `Box`. Both forward and mutual refs must resolve to B's OWN
 // nominal TypeIds, not the first same-name authors in A.
-Box  :: struct { y: s64; peer: *Node; }
-Node :: struct { m: s64; owner: *Box; }
-b_test :: () -> s64 {
+Box  :: struct { y: i64; peer: *Node; }
+Node :: struct { m: i64; owner: *Box; }
+b_test :: () -> i64 {
     nd := Node.{ m = 99, owner = null };
     bx := Box.{ y = 7, peer = @nd };
     // Reads B's Node.m (99); pre-fix the forward ref bound to A's Node (which has