Per mara's review on the side-panel PR: create the shared <hive-side-panel>
instance once at module-evaluation time instead of lazily on first call
via an ensurePanel() guard every wrapper method had to remember to call.
ES modules execute after the document is parsed (same timing as a defer
script), so document.body is already available when this code runs --
lazy init bought nothing here and left a footgun for any future method
added to either wrapper.
996899fcad