test(ffi-linkage): Phase 5.0 prereq — xfail extern C-variadic tail loses its ...

A trailing `..args: []T` on an `extern` fn must map to the C `...` tail
like its `#foreign` twin (example 1218). Today the variadic handling in
both declareFunction (is_variadic drop) and packVariadicCallArgs
(call-site early-out) is gated on `#foreign` only, so a variadic
`extern` keeps the trailing slice param and slice-packs the extras —
garbage at the C ABI (probe: sum_ints(3,10,20,30) → 53316585, not 60).

Example 1229 pins the DESIRED correct output; the next commit extends
both gates to cover extern and greens it. Prerequisite for migrating the
fn-decl `#foreign` path onto `extern`.

645 corpus (1229 xfail), 444 unit.

examples/1229-ffi-extern-cvariadic.c

@@ -0,0 +1,20 @@
+#include <stdarg.h>
+
+long long sx_ext_sum_ints(int n, ...) {
+    va_list ap;
+    va_start(ap, n);
+    long long total = 0;
+    for (int i = 0; i < n; i++) total += va_arg(ap, int);
+    va_end(ap);
+    return total;
+}
+
+double sx_ext_avg_doubles(int n, ...) {
+    va_list ap;
+    va_start(ap, n);
+    double total = 0.0;
+    for (int i = 0; i < n; i++) total += va_arg(ap, double);
+    va_end(ap);
+    if (n == 0) return 0.0;
+    return total / n;
+}

examples/1229-ffi-extern-cvariadic.sx

@@ -0,0 +1,27 @@
+// `extern` C-variadic tail: a trailing `..args: []T` on an `extern` fn
+// maps to the C calling convention's `...`, exactly like its `#foreign`
+// twin (example 1218). Extras at the call site pass through the variadic
+// slot with standard default argument promotion (i8/i16/bool → i32,
+// f32 → f64), NOT packed into an sx slice.
+//
+// Regression (FFI-linkage Part B): the `is_variadic` drop in
+// `declareFunction` + the call-site early-out in `packVariadicCallArgs`
+// were gated on `#foreign` only, so a migrated variadic `extern` lost
+// its `...` tail and slice-packed the extras (garbage at the C ABI).
+
+#import "modules/std.sx";
+
+#import c {
+    #source "1229-ffi-extern-cvariadic.c";
+};
+
+sx_ext_sum_ints    :: (n: i32, ..args: []i32) -> i64 extern;
+sx_ext_avg_doubles :: (n: i32, ..args: []f64) -> f64 extern;
+
+main :: () -> i32 {
+    print("sum_ints(3, 10, 20, 30) = {}\n", sx_ext_sum_ints(3, 10, 20, 30));
+    print("sum_ints(0)             = {}\n", sx_ext_sum_ints(0));
+    print("avg_doubles(2)          = {}\n", sx_ext_avg_doubles(2, 1.5, 2.5));
+    print("avg_doubles(3)          = {}\n", sx_ext_avg_doubles(3, 1.0, 2.0, 3.0));
+    0
+}

examples/expected/1229-ffi-extern-cvariadic.exit

@@ -0,0 +1 @@
+0

examples/expected/1229-ffi-extern-cvariadic.stderr

@@ -0,0 +1 @@
+

examples/expected/1229-ffi-extern-cvariadic.stdout

@@ -0,0 +1,4 @@
+sum_ints(3, 10, 20, 30) = 60
+sum_ints(0)             = 0
+avg_doubles(2)          = 2.000000
+avg_doubles(3)          = 2.000000