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.

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(" ");

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;
 }
 

examples/50-smoke.sx

@@ -63,7 +63,7 @@ pair_add :: (a: $T, b: $U) -> s64 {
     cast(s64) a + cast(s64) b;
 }
 
-typed_sum :: (args: ..s32) -> s32 {
+typed_sum :: (..args: []s32) -> s32 {
     result := 0;
     for args: (it) { result = result + it; }
     result;

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));

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);";
 }