[yawaflua]

On speakers without capsule-mini-tweaker the AirPlay bundle leaves shairport-sync without metadata and with its own software volume. The airplay plugin then sees nothing: the ring stays dark, and the phone's slider attenuates the stream inside shairport-sync while the speaker's volume (shown on the display and in the panel) never changes.

• install: after the bundle's installer, enable the metadata pipe (/tmp/shairport-sync-metadata) and ignore_volume_control in /etc/shairport-sync.conf, then restart shairport-sync. An already tuned config is left alone.
• agent: send speaker plugins a new volume event (level, muted) on every volume change: voice, keys, panel, plugins. Plugins subscribe by kind, so existing ones are unaffected; documented in README and sdk/README.
• airplay plugin (1.2.0):
  • phone slider → volume.set on the speaker (pvol metadata, −30…0 dB ↔ 0…100%, −144 = mute);
  • speaker volume → phone slider via shairport-sync's SetAirplayVolume (dbus-send), without echoing a change back to where it came from;
  • dim ring while a phone has the speaker chosen but is silent;
  • with /run/capsule-controls.sock present the bundle's ap2-bridge already syncs the volume, so the plugin only lights the ring. Works together with agent: optional Capsula Mini volume via capsule-ipc #15.

Tests: installer config tuning, dB ↔ level round trip for every level, echo suppression, the bridged case. gofmt, go vet, go test -race, node --check pass.

Not tested on a real speaker yet.