Observed evaluation handles

Browser and notebook hosts can evaluate a retained reactive output once and mount it without duplicating RiX’s reactive-read detection:

const result = parseAndEvaluateObserved(source, options);
const disposeWidgets = mountOutputWidgets(root, result.value, {
  observe: result.observe,
});

// When the host removes the output:
disposeWidgets();
result.dispose();

parseAndEvaluateObserved and parseAndEvaluateObservedAsync return { value, observe, dispose }. observe is null for ordinary values and for derived expressions that do not return one unambiguous reactive source. A listener receives (nextValue, event). Its unsubscribe function and the handle’s dispose() are idempotent; neither disposes the underlying reactive identity owned by the RiX context.

Hosts that already parse and lower source can use evaluateObserved or evaluateObservedAsync for one IR node. Their optional selectValue callback supports host publication commands whose displayed value is selected during evaluation rather than returned by the command itself.