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/0755-modules-same-name-struct-ambiguous/a.sx

@@ -1,2 +1,2 @@
 // One of two flat-imported authors of a same-name `Thing`.
-Thing :: struct { a: s64; }
+Thing :: struct { a: i64; }

examples/0755-modules-same-name-struct-ambiguous/b.sx

@@ -1,3 +1,3 @@
 // The second flat-imported author of a same-name `Thing`. With both visible and
 // no own author in `main`, a bare `Thing` reference is genuinely ambiguous.
-Thing :: struct { b: s64; }
+Thing :: struct { b: i64; }