Add runtime event client boundary for web React

[enricopiovesan]

Purpose

Implement the thin client boundary that lets React start Traverse workflows and subscribe to correlated runtime events, without importing any private Traverse internals.

Definition of Done

• React can start a workflow through a public Traverse runtime/client boundary
• React subscribes to correlated runtime events
• UI state is driven by events
• Failure, loading, progress, and final states are visible in the UI
• No private Traverse internals imported

Dependencies

Depends on #2 (UI shell scaffold).