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/heads.go

@@ -7,6 +7,7 @@ import (
 	"math"
 	"os"
 	"path/filepath"
+	"sync"
 
 	ort "github.com/yalue/onnxruntime_go"
 )
@@ -54,9 +55,12 @@ const (
 type RouterHeads struct {
 	tokenizer *unigramTokenizer
 	session   *ort.DynamicSession[int64, float32]
+	runtime   *ONNXRuntimeLease
 	intents   []Intent
 	sources   []Source
 	threshold float64
+	closeOnce sync.Once
+	closeErr  error
 }
 
 // headsMeta — router_heads.json, written beside the weights by the exporter.
@@ -70,8 +74,9 @@ type headsMeta struct {
 // NewRouterHeads loads the graph and its label order. modelPath points at the
 // .onnx; the external weights and router_heads.json sit beside it.
 //
-// It assumes the ONNX environment is already initialised, because the embedder
-// does that at startup and the runtime allows it once.
+// It shares the process-global ONNX environment with the embedder. The runtime
+// lease is independent so shutdown order cannot unload the library while this
+// graph's session is still alive.
 func NewRouterHeads(modelPath, tokenizerPath string) (*RouterHeads, error) {
 	metaPath := filepath.Join(filepath.Dir(modelPath), "router_heads.json")
 	raw, err := os.ReadFile(metaPath)
@@ -106,18 +111,24 @@ func NewRouterHeads(modelPath, tokenizerPath string) (*RouterHeads, error) {
 	if err != nil {
 		return nil, fmt.Errorf("heads: tokenizer: %w", err)
 	}
+	runtime, err := AcquireONNXRuntime("")
+	if err != nil {
+		return nil, fmt.Errorf("heads: %w", err)
+	}
 	session, err := ort.NewDynamicSession[int64, float32](
 		modelPath,
 		[]string{"input_ids", "attention_mask"},
 		[]string{"intent", "source", "slots", "clarify"},
 	)
 	if err != nil {
+		_ = runtime.Close()
 		return nil, fmt.Errorf("heads: create session: %w", err)
 	}
 
 	return &RouterHeads{
 		tokenizer: tok,
 		session:   session,
+		runtime:   runtime,
 		intents:   intents,
 		sources:   sources,
 		threshold: headsThreshold,
@@ -128,8 +139,14 @@ func (h *RouterHeads) Close() error {
 	if h == nil {
 		return nil
 	}
-	h.session.Destroy()
-	return nil
+	h.closeOnce.Do(func() {
+		if h.session != nil {
+			h.session.Destroy()
+			h.session = nil
+		}
+		h.closeErr = h.runtime.Close()
+	})
+	return h.closeErr
 }
 
 // headsResult — one forward pass, read back.