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/ffi-foreign-cvariadic.sx

@@ -1,4 +1,4 @@
-// `#foreign` C-variadic tail: trailing `args: ..T` on a foreign fn maps
+// `#foreign` C-variadic tail: trailing `..args: []T` on a foreign fn maps
 // to the C calling convention's `...`. Extras at the call site are
 // passed via the variadic slot with the standard default argument
 // promotion (s8/s16/bool → s32, f32 → f64) applied implicitly.
@@ -9,9 +9,9 @@
     #source "ffi-foreign-cvariadic.c";
 };
 
-sx_ffi_sum_ints     :: (n: s32, args: ..s32) -> s64 #foreign;
-sx_ffi_avg_doubles  :: (n: s32, args: ..f64) -> f64 #foreign;
-sx_ffi_count_args   :: (tag: *u8, args: ..*u8) -> s32 #foreign;
+sx_ffi_sum_ints     :: (n: s32, ..args: []s32) -> s64 #foreign;
+sx_ffi_avg_doubles  :: (n: s32, ..args: []f64) -> f64 #foreign;
+sx_ffi_count_args   :: (tag: *u8, ..args: []*u8) -> s32 #foreign;
 
 main :: () -> s32 {
     print("sum_ints(3, 10, 20, 30) = {}\n", sx_ffi_sum_ints(3, 10, 20, 30));