maven: embedder config validation + docs (task 1)

- Tests: all-three-set → ok; one-missing → error; nil → ok
- Log message on HashEmbedder fallback in voice.go
- Document floor mode in START.md

Co-Authored-By: opencode <opencode@anthropic.com>

cmd/mavend/voice.go

@@ -156,6 +156,7 @@ func wireVoice(cfg *config.Config, coreAPI ipc.CoreAPI, phr phraser.Phraser) (*v
 		log.Printf("voice: onnx embedder loaded (%d dim)", onnx.Dim())
 		emb = onnx
 	} else {
+		log.Printf("voice: embedder not configured, using HashEmbedder floor")
 		emb = router.NewHashEmbedder(1024)
 	}
 	w.embedder = emb