Keep Radio playback state consistent after a silent Next

Review follow-ups for HAL-722. A Next or Previous that only selects a
station now leaves nothing of the previous stream behind:

- Selecting another station without playing stops a paused or failed
  stream, so the panel, Stop, voice summaries and the audio element agree
  on "stopped". A fallback the caller armed for the new selection is kept.
- A silent UI step arms no broadcaster fallback, and a user Play of the
  selected station drops one left by a cancelled voice step. Voice steps
  pass `fallback: true` because voice starts the prepared playback later.
- A user step to another station emits a `select` playback control, and
  the realtime voice handoff treats it like Pause and Stop.
- A tuner release onto a station the directory no longer has stops the
  stream the drag paused, publishing one state with the gesture's origin,
  so the button reads PLAY and a voice handoff is cancelled.
- The transport buttons offer Pause while a stream connects, matching
  what a click does.
- The privacy note names every action that connects to a broadcaster
  (play, finishing tuning, choosing a station on the globe) in all
  locales.

The Next/Previous layer tests live in src/data/radioNextState.test.mjs.

src/layers/radio/playback.js

@@ -344,7 +344,12 @@ export function createPlayback({ state: layerState, services, parts, source }) {
     }
     if (!parts.interaction.radioPresentationAllowed())
       return Promise.resolve(false);
-    if (!layerState._selectedId) {
+    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.
+      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;