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/testSupport/radioLayerHarness.mjs

@@ -0,0 +1,101 @@
+/** Shared Radio layer test stand-ins: a station row, fake Audio, directory fetch and viewer. */
+import { radioLayer } from '../data/radio.js';
+
+/** One accepted Radio Browser directory row. */
+export function radioStationRow(overrides = {}) {
+  return {
+    id: '00000000-0000-4000-8000-000000000001',
+    name: 'Station',
+    lat: 30,
+    lon: -97,
+    streamUrl: 'https://radio.example.com/station.mp3',
+    homepage: null,
+    tags: ['news'],
+    languages: ['English'],
+    state: 'Texas',
+    country: 'United States',
+    countryCode: 'US',
+    metadataTrust: 'untrusted-community',
+    codec: 'MP3',
+    bitrate: 128,
+    ...overrides,
+  };
+}
+
+/**
+ * Run `body` against an enabled `radioLayer` that serves `stations`.
+ * Streams listed in `failingUrls` (mutable) reject play(); `body` receives the
+ * played URLs and the catalog, whose rows and generation the next update() serves.
+ */
+export async function withRadioLayer({ stations, failingUrls = [] }, body) {
+  const originalFetch = globalThis.fetch;
+  const originalAudio = globalThis.Audio;
+  const playUrls = [];
+  const catalog = { rows: stations, generation: 1 };
+  globalThis.Audio = class FakeAudio {
+    constructor() {
+      this.volume = 0.8;
+      this.src = '';
+    }
+
+    addEventListener() {}
+    pause() {}
+    play() {
+      playUrls.push(this.src);
+      if (failingUrls.includes(this.src)) {
+        return Promise.reject(
+          new DOMException('unavailable', 'NotSupportedError'),
+        );
+      }
+      return Promise.resolve();
+    }
+    removeAttribute() {
+      this.src = '';
+    }
+    load() {}
+  };
+  globalThis.fetch = async (url) =>
+    String(url).startsWith('/api/radio/click/')
+      ? { ok: true }
+      : new Response(
+          JSON.stringify({
+            stations: catalog.rows,
+            stale: false,
+            degraded: false,
+            acceptedGeneration: catalog.generation,
+            catalogInstance: 'qa-instance-a',
+            updatedAt: new Date().toISOString(),
+          }),
+          { status: 200, headers: { 'Content-Type': 'application/json' } },
+        );
+  const viewer = {
+    camera: { positionWC: { x: 7_000_000, y: 0, z: 0 } },
+    scene: {
+      canvas: {
+        disableRootEvents: true,
+        onwheel: null,
+        addEventListener() {},
+        removeEventListener() {},
+      },
+    },
+    dataSources: { add() {}, remove() {} },
+    entities: {
+      add(entity) {
+        return entity;
+      },
+      remove() {},
+    },
+  };
+  radioLayer.destroy();
+  try {
+    radioLayer.init(viewer);
+    radioLayer.enable();
+    await radioLayer.update();
+    await body({ playUrls, catalog });
+  } finally {
+    globalThis.fetch = originalFetch;
+    if (originalAudio === undefined) delete globalThis.Audio;
+    else globalThis.Audio = originalAudio;
+    radioLayer.destroy();
+  }
+}