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.

docs/CURRENT-STATE.md

@@ -4774,8 +4774,7 @@ result reads enabled state from the manager and audio state from the Radio
 module. Voice Pause is a playback-only no-op while the layer is disabled; it
 never enables Radio and reports a fulfilled-false pause as failure. A
 newer-response or user-origin Stop still cancels stale handoff work, as do a
-user Pause, a user Previous/Next that only selects another station, and a tuner
-release onto a station the directory no longer has.
+user Pause and a user Previous/Next that only selects another station.
 Only a newer user turn or session teardown aborts the complete active-tool set.
 A cancelled turn cannot publish a late Radio playback request. Dedicated Radio
 controls and generic layer-visibility commands both forward cancellation.