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/19-varargs.sx

@@ -1,6 +1,6 @@
 #import "modules/std.sx";
 
-sum :: (args: ..s32) -> s32 {
+sum :: (..args: []s32) -> s32 {
     result := 0;
     for args: (it) {
         result = result + it;
@@ -8,7 +8,7 @@ sum :: (args: ..s32) -> s32 {
     result;
 }
 
-print_all :: (args: ..s32) {
+print_all :: (..args: []s32) {
     for args: (it) {
         out(int_to_string(it));
         out(" ");