Give the daemon a heads_path and a fixture arm (V-664)

embedder.heads_path is empty by default and deploy/mavend.json sets
it. A missing or broken weights file logs and leaves the heads nil,
because refusing to start over a routing accelerator would trade a
working box for a better one.

TestONNXRoutingHeads is the same cascade TestONNXBaseline scores with
one arm added, so the two are directly comparable. It also checks the
Go tokenizer against the Python one, since the heads were trained
through transformers and are read through a hand-written tokenizer: a
mismatch shows up here as a score below what Python measured on the
same weights, and nowhere else. That is how the reversed word pieces
were found.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013ptwopxyo3Z2kwFckHkLvN

internal/config/voice.go

@@ -37,6 +37,16 @@ type EmbedderConfig struct {
 	ModelPath     string `json:"model_path,omitempty"`
 	TokenizerPath string `json:"tokenizer_path,omitempty"`
 	LibPath       string `json:"lib_path,omitempty"`
+
+	// HeadsPath — the routing heads graph, which is a fine-tuned COPY of the
+	// model above with four linear heads on its pooled output (V-664). Empty
+	// means no heads, and the cascade runs exactly as it did before they
+	// existed. It shares LibPath and TokenizerPath, and router_heads.json is
+	// read from the same directory.
+	//
+	// It must never be pointed at ModelPath. Memory recall depends on the
+	// resident copy scoring what it scored, and the fine-tuned one does not.
+	HeadsPath string `json:"heads_path,omitempty"`
 }
 
 // WeatherConfig configures the weather provider for voice queries.