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/0722-modules-flat-same-name-own/a.sx

@@ -1,5 +1,5 @@
 // a.sx authors `greet`. Its own `from_a` calls `greet` bare — under fix-0102c
 // that binds a.sx's OWN author (own-author wins), even though b.sx also
 // authors `greet` and the first-wins merge keeps only one in the merged scope.
-greet  :: () -> s64 { return 1; }
-from_a :: () -> s64 { return greet(); }
+greet  :: () -> i64 { return 1; }
+from_a :: () -> i64 { return greet(); }

examples/0722-modules-flat-same-name-own/b.sx

@@ -1,4 +1,4 @@
 // b.sx authors its OWN `greet`. `from_b`'s bare `greet` must bind b.sx's
 // author (2), not the first-wins winner from a.sx.
-greet  :: () -> s64 { return 2; }
-from_b :: () -> s64 { return greet(); }
+greet  :: () -> i64 { return 2; }
+from_b :: () -> i64 { return greet(); }