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/ui/radioPresentation.playbackError.test.mjs

@@ -22,7 +22,6 @@ function render(audioError) {
     destroyed: false,
     _radioPanel: el(),
     _radioPlaybackState: el(),
-    _syncContextRadioLauncherState() {},
     actions: {
       getLifecycle: () => null,
       preservePanelStateDuringClear: () => false,