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/0544-packs-imported-pack-fn-fixed-param-source-pin/dep.sx

@@ -1,3 +1,3 @@
 // `Needs` lives two flat-import hops away from `main` (main -> lib -> dep), so
 // it is NOT bare-visible at the call site under non-transitive visibility.
-Needs :: struct { v: s64; }
+Needs :: struct { v: i64; }

examples/0544-packs-imported-pack-fn-fixed-param-source-pin/lib.sx

@@ -9,8 +9,8 @@ make :: () -> Needs => Needs.{ v = 7 };
 
 // Control: a plain (non-pack) fn with the same fixed param already resolves in
 // its defining module — the cross-module call-arg typing path is source-pinned.
-use_plain :: (n: Needs) -> s64 => n.v;
+use_plain :: (n: Needs) -> i64 => n.v;
 
 // Pack fn: the fixed-prefix param `n: Needs` is bound during pack
 // monomorphization. Its type must resolve under this module's source.
-use_pack :: (n: Needs, ..$args) -> s64 => n.v + args[0];
+use_pack :: (n: Needs, ..$args) -> i64 => n.v + args[0];