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/0831-modules-namespace-alias-carry/rich.sx

@@ -1,9 +1,9 @@
 Thing :: struct {
-    v: s64;
+    v: i64;
     init :: () -> Thing { Thing.{ v = 5 } }
-    get :: (self: *Thing) -> s64 { self.v }
+    get :: (self: *Thing) -> i64 { self.v }
 }
 Color :: enum { red; green; }
-LIMIT :s64: 99;
+LIMIT :i64: 99;
 Box :: struct ($T: Type) { item: T; }
-helper :: () -> s64 { 7 }
+helper :: () -> i64 { 7 }