ffi 2.0: xfail parser test for Foo :: #jni_class(...) { ... }

Today's parser doesn't recognize #jni_class as a hash directive
after `::`, so it falls through to expression parsing and errors
at the `#` token. Step 2.1 extends parseConstBinding to accept
the directive (opaque on empty body) and re-snapshots this file
to green.

examples/ffi-jni-class-01-empty.sx

@@ -0,0 +1,18 @@
+// Phase 2 step 2.0 (PLAN-FFI.md): xfail parser test for the
+// type-introducer `Foo :: #jni_class("java/path/Foo") { }` directive.
+//
+// Today's parser doesn't recognize `#jni_class` as a known hash
+// directive after `::`, so it falls through to expression parsing
+// and fails at the `#` token. Step 2.1 (next commit) extends
+// `parseConstBinding` to accept the directive, treat an empty body
+// as an opaque forward declaration, and re-snapshots this file to
+// capture the green behavior.
+
+#import "modules/std.sx";
+
+Foo :: #jni_class("java/path/Foo") { }
+
+main :: () -> s32 {
+    print("parse-only ok\n");
+    0;
+}

tests/expected/ffi-jni-class-01-empty.exit

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

tests/expected/ffi-jni-class-01-empty.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/ffi-jni-class-01-empty.sx:13:8: error: unexpected token in expression