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.

library/modules/fs.sx

@@ -24,11 +24,11 @@ libc :: #library "c";
 // API below wraps them. Users should not call these directly.
 //
 // macOS `open` is variadic in C (`int open(const char*, int, ...)`);
-// declared with `args: ..s32` so the mode is passed via the C
+// declared with `..args: []s32` so the mode is passed via the C
 // variadic tail. Without that, the mode arg goes to the wrong
 // register on arm64 and the file ends up with mode 0.
 
-open   :: (path: [:0]u8, flags: s32, args: ..s32) -> s32 #foreign libc;
+open   :: (path: [:0]u8, flags: s32, ..args: []s32) -> s32 #foreign libc;
 close  :: (fd: s32) -> s32 #foreign libc;
 read   :: (fd: s32, buf: [*]u8, count: usize) -> isize #foreign libc;
 write  :: (fd: s32, buf: [*]u8, count: usize) -> isize #foreign libc;

library/modules/std.sx

@@ -384,12 +384,12 @@ build_format :: (fmt: string) -> string {
     code;
 }
 
-format :: ($fmt: string, args: ..Any) -> string {
+format :: ($fmt: string, ..args: []Any) -> string {
     #insert build_format(fmt);
     #insert "result;";
 }
 
-print :: ($fmt: string, args: ..Any) {
+print :: ($fmt: string, ..args: []Any) {
     #insert build_format(fmt);
     #insert "out(result);";
 }