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/ui/radioBindings.js

@@ -328,8 +328,7 @@ export function bindRadioControls() {
     const pool = this._radioTunerPool.length
       ? this._radioTunerPool
       : this._radioTunerStations;
-    // Keep the step silent unless a stream is active. An active stream follows
-    // the selection, so the station label matches the broadcaster.
+    // Silent unless a stream is active; an active stream follows the selection.
     const { audioState } = this.radio.getUIState();
     const cycled = this.radio.cycleStation(direction, {
       rotate,

src/ui/radioPresentation.js

@@ -20,9 +20,7 @@ export function renderRadioState(state) {
   const interactive = enabled && !transitioning && !uncertain;
   const selected = state.selected || null;
   const hasStations = state.filteredCount > 0;
-  const activePlayback = ['loading', 'buffering', 'playing'].includes(
-    state.audioState,
-  );
+  const activePlayback = ['playing', 'buffering'].includes(state.audioState);
   document
     .getElementById('title-bar')
     ?.classList.toggle('radio-broadcasting', state.audioState === 'playing');