State management and thread/message access layer for assistant-ui chat applications.

• Provides three runtime types: AssistantRuntime for overall state, ThreadRuntime for current conversation, and ThreadListRuntime for managing multiple threads
• Access state via useAuiState selectors and perform operations through useAui() API (append messages, cancel runs, edit/reload messages, query capabilities)
• Emit and listen to lifecycle events: thread.runStart, thread.runEnd, composer.send, and thread.modelContextUpdate
• Hierarchical runtime structure supports per-message and per-content-part operations, with built-in capability detection for cancel, edit, reload, copy, speak, and attachments