Main records which client surface started each thread and turn; the v2
dispatch path replaced the v1 handler that did the recording, so v2
environments only reported connections. The v2 RPC layer now records
client.thread.started on thread launches (plus client.turn.requested when
the launch carries an initial message) and client.turn.requested on
message dispatches, using the connection's announced origin. Recording is
best-effort — attribution can never fail the user's command.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>