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.

changelog.d/1474-radio-next-no-autoplay.md

@@ -1,5 +1,2 @@
 - Radio Next and Previous no longer start a broadcaster stream before Play is
-  pressed: they only select a station, and once a stream is playing it follows
-  the new station. Stepping away from a paused or failed stream releases it, the
-  Play button offers Pause while a stream connects, and the privacy note names
-  every action that connects to a broadcaster (gekh, #1474).
+  pressed; while a stream plays, it follows the new station (gekh, #1474).