phraser, mavend: read the fallbacks from the file (V-501)

The accessors are functions now, so the call sites that compared against one
literal compare against the entry instead: IsUnknownFallback and
IsSourcesFallback in the daemon tests, the entry key in the phraser tests. A
reworded variant no longer breaks a Go test.

The eval scores every variant on the persona checks the nudges already pass.

cmd/mavend/actions.go

@@ -66,7 +66,7 @@ func (h *reactiveHandler) actionChat(ctx context.Context, dec router.Decision) s
 		log.Printf("voice: chat: %v", err)
 	}
 	if reply == "" {
-		return phraser.ChatFallback
+		return phraser.ChatFallback()
 	}
 	return reply
 }