Scheduling: batch, flushSync, untrack

Writes mark dependents stale and schedule one flush in a microtask, so any number of writes in the same task cause one run of each dependent. batch and flushSync run the flush at the end of the callback instead of waiting. untrack reads without subscribing.

Use untrack inside an effect for a value you need but must not react to; callbacks you pass to ShadowJS APIs (onMount, list row functions, event handlers) already run untracked.