Refuse a heads_path that is the embedder's own model file (V-692)

CLAUDE.md, internal/config/voice.go and docs/routing.md all say the routing
heads graph is a fine-tuned copy of the embedder, never the embedder's own file.
Nothing enforced it. The daemon loaded whatever the key pointed at, so pointing
both keys at one file cost recall with no error and no log line, which reads as
ordinary drift rather than as a misconfiguration.

validateVoice now refuses it at load. Both paths are cleaned and made absolute
first, so "./m.onnx" and "$PWD/m.onnx" are one path, and then compared with
os.SameFile, which catches a copy that is a symlink or a hard link. A path that
does not stat is left to the loader, whose error message is better than this
check can give.

Refusing to start is deliberate and it differs from the loader's treatment of a
broken weights file, which logs and leaves the heads nil on purpose. That case
is a missing accelerator. This one is a working file in the wrong role, and a
daemon that cannot route well should say so rather than answer worse.

deploy/mavend.json points the two keys at different files, so the live config
still starts.

internal/config/voice.go

@@ -2,6 +2,9 @@ package config
 
 import (
 	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
 	"time"
 )
 
@@ -141,10 +144,55 @@ func (c *Config) validateVoice() error {
 		if e.ModelPath == "" || e.TokenizerPath == "" || e.LibPath == "" {
 			return errors.New("voice.embedder: all three of model_path, tokenizer_path, lib_path must be set, or remove embedder to use the floor stub")
 		}
+		if err := e.checkHeadsDistinct(); err != nil {
+			return err
+		}
 	}
 	return nil
 }
 
+// checkHeadsDistinct refuses a heads graph that is the embedder's own file
+// (V-692). The rule is stated on HeadsPath above and in CLAUDE.md, and until
+// now nothing enforced it: the daemon loaded whatever the key pointed at, so
+// pointing both keys at one file cost recall with no error and no log line. It
+// reads as ordinary drift, which is the worst kind of misconfiguration.
+//
+// Refusing to start is the right trade here. The heads are an accelerator and a
+// broken weights file is deliberately not fatal in voicewire.go, but this is not
+// a broken file. It is a working file in the wrong role, and a daemon that
+// cannot route well should say so rather than answer worse.
+//
+// Cleaned and made absolute first, so "./m.onnx" and "$PWD/m.onnx" are one
+// path. Then SameFile, which catches the copy that is a symlink or a hard link
+// to the original. A path that does not stat is left to the loader, which fails
+// on it with a better message than this can give.
+func (e *EmbedderConfig) checkHeadsDistinct() error {
+	if e.HeadsPath == "" || e.ModelPath == "" {
+		return nil
+	}
+	heads, model := absClean(e.HeadsPath), absClean(e.ModelPath)
+	same := heads == model
+	if !same {
+		hi, herr := os.Stat(heads)
+		mi, merr := os.Stat(model)
+		same = herr == nil && merr == nil && os.SameFile(hi, mi)
+	}
+	if same {
+		return fmt.Errorf("voice.embedder: heads_path and model_path are the same file (%s) — the heads graph is a fine-tuned copy, and scoring recall with it degrades what the resident embedder already stored", heads)
+	}
+	return nil
+}
+
+// absClean — the comparable form of a path. Abs fails only when the working
+// directory is unreadable, and a cleaned relative path is still worth comparing,
+// so the error falls back rather than propagating.
+func absClean(p string) string {
+	if abs, err := filepath.Abs(p); err == nil {
+		return abs
+	}
+	return filepath.Clean(p)
+}
+
 // VoiceConfig — the client↔core TCP surface + the stt/tts worker-module
 // seams.
 //

internal/config/voice_test.go

@@ -0,0 +1,70 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// The heads graph is a fine-tuned copy of the embedder, and pointing both keys
+// at one file degrades recall with no error and no log line (V-692). These are
+// the shapes that used to boot clean.
+func TestHeadsPathMustNotBeTheModelFile(t *testing.T) {
+	dir := t.TempDir()
+	model := filepath.Join(dir, "model.onnx")
+	heads := filepath.Join(dir, "heads.onnx")
+	for _, p := range []string{model, heads} {
+		if err := os.WriteFile(p, []byte("onnx"), 0o600); err != nil {
+			t.Fatalf("write %s: %v", p, err)
+		}
+	}
+	link := filepath.Join(dir, "link.onnx")
+	if err := os.Symlink(model, link); err != nil {
+		t.Fatalf("symlink: %v", err)
+	}
+
+	// A path that stats and one that does not, because the guard compares the
+	// cleaned string before it stats anything.
+	for name, headsPath := range map[string]string{
+		"the same path":     model,
+		"a symlink to it":   link,
+		"an uncleaned path": filepath.Join(dir, ".", "sub", "..", "model.onnx"),
+		"a path on no disk": filepath.Join(dir, "absent.onnx"),
+	} {
+		t.Run(name, func(t *testing.T) {
+			same := headsPath != filepath.Join(dir, "absent.onnx")
+			err := voiceConfigWith(t, model, headsPath)
+			if same && err == nil {
+				t.Fatal("want a startup error, got a daemon that routes worse in silence")
+			}
+			if same && !strings.Contains(err.Error(), "heads_path") {
+				t.Fatalf("the error does not name the key: %v", err)
+			}
+			if !same && err != nil {
+				t.Fatalf("a distinct heads_path was refused: %v", err)
+			}
+		})
+	}
+
+	if err := voiceConfigWith(t, model, heads); err != nil {
+		t.Fatalf("two distinct files were refused: %v", err)
+	}
+	if err := voiceConfigWith(t, model, ""); err != nil {
+		t.Fatalf("no heads at all was refused: %v", err)
+	}
+}
+
+// voiceConfigWith loads a minimal enabled voice block through the real Load, so
+// the test exercises the startup path and not just the check in isolation.
+func voiceConfigWith(t *testing.T, model, heads string) error {
+	t.Helper()
+	body := `{"voice":{"enabled":true,"bind":"127.0.0.1:9100","embedder":{` +
+		`"model_path":"` + model + `","tokenizer_path":"/t.json","lib_path":"/l.so"`
+	if heads != "" {
+		body += `,"heads_path":"` + heads + `"`
+	}
+	body += `}}}`
+	_, err := Load(writeConfig(t, body))
+	return err
+}