ffi issue-0049: new-form variadic cross-module LLVM crash — xfail lock-in

Migrating stdlib's `path_join` to the new variadic syntax
(`(..parts: []string) -> string`) surfaces a latent compiler bug:
`resolveParamType` and `packVariadicCallArgs` treat the new-form
declaration the same as the legacy `parts: ..string` and wrap the
element type in `sliceOf` regardless of whether it already is one.
The new form's `[]string` becomes `[][]string`; the call-site
marshal pack emits `[N x string]` (correct) but the callee stores
its slice param into a `[]([]string)`-typed slot. The shape
mismatch propagates as null/undef Refs that crash
`LLVMBuildExtractValue` inside `emitStrCmp` during emission.

`examples/121-ios-sim-bundle.sx` (existing) and the new focused
`examples/174-new-form-variadic-cross-module.sx` both fail today
with the segfault. The next commit fixes `resolveParamType` +
`packVariadicCallArgs` so both flip green. Stdlib's `format` /
`print` / `open` and the example fixtures stay on the legacy form
in this commit — they migrate in the follow-up cleanup commit.

examples/174-new-form-variadic-cross-module.sx

@@ -0,0 +1,28 @@
+// Regression: new-form variadic `..name: []T` defined in an imported
+// module is callable from another module without crashing LLVM emit.
+//
+// Before the fix in `resolveParamType` + `packVariadicCallArgs`,
+// the new-form variadic's element type went through one extra
+// `sliceOf` wrap (the helpers treated `..parts: []string` the same
+// as the legacy `parts: ..string` and added a slice level on top
+// of the already-declared slice). The double-wrapped `[][]T`
+// signature mismatched what the call-site marshalling emitted as
+// `[N x T]`, producing null/undef Refs that crashed
+// `LLVMBuildExtractValue` inside `emitStrCmp` during emission.
+//
+// Today's stdlib `path_join` uses the new form
+// (`(..parts: []string) -> string`); it lives in `modules/std.sx`
+// and is called here from the test module. Two- and three-arg
+// shapes must round-trip the slice through the function-call
+// boundary and concatenate the parts with '/'. Empty join (no
+// args) returns "".
+
+#import "modules/std.sx";
+
+main :: () -> s32 {
+    print("{}\n", path_join());
+    print("{}\n", path_join("a"));
+    print("{}\n", path_join("a", "b"));
+    print("{}\n", path_join("a", "b", "c", "d"));
+    return 0;
+}