Implement the DIRECT connection-initiation mode so a download can target a
peer the client is not already connected to:
- Fix the outbound peer handshake. on_connection_established sent
build_watch_user() (a server WatchUser message, wrongly framed with
get_data()) for outbound connections; it now sends a proper length-prefixed
PeerInit (peer code 1) with our own username. own_username is plumbed in
via PeerRegistry -> PeerActor; inbound peers (outbound=false) are unchanged.
- download() now auto-connects: if there is no control connection to the peer
it sends GetPeerAddress and defers the QueueUpload until the outbound stream
handshakes, signalled by a new ClientOperation::PeerConnected. A never-
connected client still resolves the download to Failed (unit test).
- Add Client::connect_peer(username) to open/pre-warm a control connection.
Verified end to end against soulfind: a_file_downloads_from_a_peer_via_direct_connection
logs a mock peer into the server, advertises its port via SetWaitPort, and the
client resolves the address, dials it, and downloads the file — bytes match.
The F-serve leg is shared with the passive-download mock.