Remote sessions add a relay transport, arrival timing, and earlier attachment preparation

What

Remote sessions are Claude Code sessions that run on a server and receive your messages over the network. This release changes how those messages are delivered and timed, and adds a new way to prepare attached files earlier.

• A new relay transport joins the ways a remote session can connect. The live event stream transport gains onDeliveredCallback and connectionSessionWriters.
• Queued messages now carry aheadOfRead and an optional serverReceivedWallMs.
• The session message format gains an optional server_received_wall_ms, the session server's clock time when it received the request. Claude Code copies it into the turn result as user_message_server_received_wall_ms.
• A new server-controlled switch, tengu_amber_ferry (built-in fallback: off), turns on an aheadOfFiles mode for incoming user messages. In that mode, preparing attachments starts before the file downloads finish, and the existing read-ahead behind tengu_polished_reef is skipped. Before, attachments were prepared by a plain call, with read-ahead controlled only by tengu_polished_reef.
• The attachment timing record gains an aheadOfFiles field, and turn timing also records aheadOfRead.

Why

These changes measure and aim to shorten the delay before a remote or cloud session starts working on a message, especially one with attached files. Nothing has been read about tengu_amber_ferry, so whether the earlier attachment preparation is in use is not known.