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/20-any-varargs.sx

@@ -6,7 +6,7 @@ Point :: struct {
 }
 
 // Print all arguments — accepts any type, dispatches via type-switch
-print_any :: (args: ..Any) {
+print_any :: (..args: []Any) {
     for args: (it) {
         type := type_of(it);
         if type == {
@@ -28,7 +28,7 @@ print_any :: (args: ..Any) {
     out("\n");
 }
 
-count :: (args: ..Any) -> s32 {
+count :: (..args: []Any) -> s32 {
     args.len;
 }