Complete Phase 9.7: Box FFI/ABI + ExternCall with working WASM demo

Co-authored-by: moe-charm <217100418+moe-charm@users.noreply.github.com>

test_wasm_simple.nyash

@@ -0,0 +1,9 @@
+# Simple WASM ExternCall test - for manual MIR injection
+static box Main {
+    init { result }
+    
+    main() {
+        me.result = "WASM test ready"
+        return me.result
+    }
+}