Trim the Radio Next/Previous change to the HAL-722 scope

- Selections take the playback fallback as a `playFallback` option and
  set it after releasing a stale stream, instead of callers arming state
  that the release then had to save and restore.
- Move the tuner `station-unavailable` release and the Pause label while
  a stream connects out of this branch: both were wrong on main already
  and go in a separate change.
- Share the Radio layer test setup in src/testSupport/radioLayerHarness.mjs
  and fold overlapping Next/Previous tests (13 to 7).
- One-line comments and a two-line changelog entry.

src/layers/radio/tuning.js

@@ -179,10 +179,7 @@ export function createTuning({ state: layerState, services, parts, source }) {
       layerState._selectionGeneration += 1;
       parts.tuningNoise.clearRadioTuningNoise({ emit: false });
       layerState._tuningUnavailableStationId = stationId;
-      // The drag paused the previous stream; with no selection left, Resume
-      // could only start the nearest station, so release it. Stop publishes
-      // the state, and as the user's gesture it cancels a voice handoff.
-      parts.playback.stopRadioPlayback({ origin });
+      parts.presentation.emitState();
       return Object.freeze({
         ok: false,
         reason: 'station-unavailable',