SDK output can hold messages back until the session start message is sent

When Claude Code is driven from a program through the Agent SDK, it writes a stream of structured messages. It can now hold some of those messages back and send them in order around the start-up message that opens the session:

• If the start-up message is sent, held messages follow straight after it.
• If it never arrives, held messages are sent just before the final result.

If writing a held message fails, Claude Code logs "held frame write failed". It also now emits a signal when a control request, a request from the controlling program, is registered.