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/playback.js

@@ -345,18 +345,16 @@ export function createPlayback({ state: layerState, services, parts, source }) {
     if (!parts.interaction.radioPresentationAllowed())
       return Promise.resolve(false);
     if (layerState._selectedId) {
-      // An explicit Play owns its attempt; a fallback left by an earlier step
-      // (a cancelled voice handoff) belongs to a playback that never started.
+      // Drop a fallback left by a voice step whose playback never started.
       layerState._playFallbackId = null;
       layerState._playFallbackFocus = null;
     } else {
       const ranked = parts.queries.rankedVisibleStations();
       if (!ranked.length) return Promise.resolve(false);
-      layerState._playFallbackId = ranked[1]?.id || null;
-      layerState._playFallbackFocus = false;
       parts.selection.selectRadioStation(ranked[0].id, {
         autoplay: false,
         focus: false,
+        playFallback: { id: ranked[1]?.id || null, focus: false },
       });
     }
     return playSelectedRadio({ origin });