ffi: migrate remaining variadic decls to new ..name: []T form

Stdlib:
- `format` / `print` in std.sx — both move from `args: ..Any` to
  `..args: []Any`. The post-issue-0049 lowering makes this safe
  across module boundaries.
- `open` in fs.sx — `args: ..s32` → `..args: []s32`. Foreign
  C-variadic semantics are preserved (the trailing `, ...` lands
  in the generated `declare` regardless of which surface form is
  used).

Examples:
- `19-varargs.sx` — `sum` / `print_all` migrated.
- `20-any-varargs.sx` — `print_any` / `count` migrated.
- `50-smoke.sx` — `typed_sum` migrated.
- `120-interp-variadic-any.sx` — comment-only update referencing
  the new form.
- `ffi-foreign-cvariadic.sx` — three C-variadic foreign decls
  migrated; header comment refreshed.

Suite stays at 214/214. The legacy `name: ..T` surface form is
still accepted by the parser; rejection follows in a later commit
once specs.md catches up.

examples/50-smoke.sx

@@ -63,7 +63,7 @@ pair_add :: (a: $T, b: $U) -> s64 {
     cast(s64) a + cast(s64) b;
 }
 
-typed_sum :: (args: ..s32) -> s32 {
+typed_sum :: (..args: []s32) -> s32 {
     result := 0;
     for args: (it) { result = result + it; }
     result;