Add Reference Runtime Spec

Why

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.

What Changes

• Create a focused follow-up change for a canonical reference-implementation-runtime spec.
• Inventory shipped and pending runtime behavior before writing normative details.
• Separate reference-specific runtime guarantees from root PDPP Collection Profile semantics.
• Defer product decisions to the follow-up implementation tasks rather than resolving them in cleanup.

Capabilities

Added Capabilities

• reference-implementation-runtime: canonical home for reference-specific runtime behavior after the polyfill connector program is ready to graduate.

Impact

• No runtime behavior changes in this stub.
• Future spec work under this change may affect reference-implementation/runtime/**, packages/polyfill-connectors/**, reference tests, and runtime documentation.