Move the changelog entry to changelog.d and clear review leftovers

The entry now lives in changelog.d/1471-context-panel-cleanup.md with
credit, as CONTRIBUTING asks, and CHANGELOG.md is back to main.

CURRENT-STATE no longer lists a compact Radio power control and scopes
the readability description to what Cyber keeps. The responsive glow
comment no longer cites the removed Radio popover, and qa-radio's
pick-buffer comment is back above settleRadioFrames.

Four test fixtures stop stubbing the removed launcher sync, and
PanelChrome drops the _syncRailPanelCollapseButton pass-through, wiring
the layout controller straight to _syncPanelCollapseButton.

src/i18n/zh-CN-display.test.mjs

@@ -88,7 +88,6 @@ test('radio option text and accessible name use the same localized parentheses',
       setLocale(locale);
       const host = {
         _radioPanel: element(), _radioFilter: element(),
-        _syncContextRadioLauncherState() {},
         actions: { getLifecycle: () => null, scheduleLayout() {} },
       };
       renderRadioState.call(host, {