Add Reference Runtime Spec

The cleanup audit found that reference-runtime behavior is proven by tests and active program work but lacks a durable canonical OpenSpec capability. Scheduler behavior, runtime validation, browser-profile binding, filesystem bindings, connector runtime logging, and inbox/notification behavior should not graduate from add-polyfill-connector-system as unbounded implementation history.

This change graduates ready reference-runtime behavior into a canonical reference-implementation-runtime OpenSpec capability. It covers behavior that is already shipped in runtime/controller/scheduler code and regression tests.

The reference runtime SHALL send each connector a START envelope containing a concrete runid, collectionmode, normalized scope, validated stream-keyed state when state persistence is enabled, and the runtime bindings available to that run.