Play announcements through the speaker's usual output (#5678)

Two resolvers picked which linked output protocol a feature is sent to, and their fallback orders disagreed, so an announcement on an idle player could land on a different interface than the one regular playback uses.

- The audio-path fallback now follows the same order as playback selection: an explicitly preferred output wins, otherwise the highest priority one.
- A preferred output only counts while it still names one of the player's own linked outputs. The stored value survives a relink, so a leftover preference could otherwise announce, or set volume and mute, on a different speaker.
- Documented why the audio-path and control-plane resolvers order differently, and why an announcement deliberately does not publish its pick as the active output protocol.