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/120-interp-variadic-any.sx

@@ -1,6 +1,6 @@
 // IR interpreter — variadic `..Any` indexing inside post-link callback.
 //
-// `format(fmt, args: ..Any)` lowers to `any_to_string(args[i])` calls.
+// `format(fmt, ..args: []Any)` lowers to `any_to_string(args[i])` calls.
 // The interpreter must be able to read every element of the packed
 // `[N x Any]` slice from within a `#run`/post-link callback, not just
 // the first two — and not just via JIT.