Make aggregate ONNX gates execute for real

Reference-count the process-global ONNX Runtime across embedder and routing-head sessions, make close idempotent, and require named proof that both aggregate routing gates executed rather than self-skipped (V-716). Owner explicitly requested direct commits to master.

internal/router/onnxruntime_test.go

@@ -0,0 +1,39 @@
+package router
+
+import (
+	"os"
+	"testing"
+)
+
+// TestONNXRuntimeCanBeReacquired pins the process-global lifecycle itself.
+// The model gates exercise live sessions; this smaller check proves that the
+// last lease unloads a managed environment and that a later gate can acquire
+// it again. It is opt-in for the same reason as every model measurement.
+func TestONNXRuntimeCanBeReacquired(t *testing.T) {
+	lib := os.Getenv("MAVEN_ONNX_LIB")
+	if lib == "" {
+		t.Skip("MAVEN_ONNX_LIB unset — see AGENTS.md § Embedder model for intent routing")
+	}
+	if _, err := os.Stat(lib); err != nil {
+		t.Skipf("missing %s: %v", lib, err)
+	}
+
+	first, err := AcquireONNXRuntime(lib)
+	if err != nil {
+		t.Fatalf("first acquire: %v", err)
+	}
+	if err := first.Close(); err != nil {
+		t.Fatalf("first close: %v", err)
+	}
+	if err := first.Close(); err != nil {
+		t.Fatalf("idempotent close: %v", err)
+	}
+
+	second, err := AcquireONNXRuntime(lib)
+	if err != nil {
+		t.Fatalf("reacquire after last close: %v", err)
+	}
+	if err := second.Close(); err != nil {
+		t.Fatalf("second close: %v", err)
+	}
+}