mountWidget centralizes both classic and shadow mounting behind a single
entry point. The shadow path attaches an open shadow root, injects CSS via
constructable stylesheets (with a deduped <style>-tag fallback for jsdom
and Safari < 16.4), and renders Preact into a stable inner wrapper so the
sibling style node is not clobbered by render() diffs.
applyStyles is exported separately so the fallback branch can be exercised
directly in unit tests without a full mount round trip — the constructable
path runs only in real browsers.
Idempotency on the host element and shadow root means repeated boots
(HMR, accidental double-load) do not duplicate DOM nodes.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
