SimulationEmbed React island with click-to-activate iframes, theme sync
via postMessage/MutationObserver, and graceful fallback when SpiceBook
is unavailable. 12 simulations across 6 books (555 timer, op-amp,
semiconductor, formulas, communications, sensor). Books without
simulations render no extra markup. client:visible hydration defers
JS cost until scrolled into view.
