Register a structured session immediately with `activity: Starting`
and run the ACP handshake in a background task. The HTTP caller gets
an instant response; the user can begin typing a prompt while the
adapter boots.
- Add `ActivityState::Starting` to the protocol
- Open the event log eagerly so the structured WS can connect during
boot (empty transcript + idle state)
- `queue_acp_prompt` stashes a prompt while `Starting`; the background
task drains it on successful handshake
- `apply_structured_activity` skips overwriting `Starting`
- `subscribe_acp_projection` allows connecting during `Starting`
- TUI/CLI map `Starting` to idle colour / "starting" label
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
