Add AckedByRemote/SentNum accessors, diffOldNum locking, and
received state list for proper old_num validation. Required for
server framebuffer diffing to track which state the client has.
Wire-compatible with C mosh. AES-128-OCB3, SSP transport, fragmentation,
protobuf state sync, latch extension fields.
Includes ServeRW bridge mode for embedding.
realrasengan