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.
Replace raw PTY passthrough with proper VT emulator processing.
PTY output feeds into charmbracelet/x/vt, snapshots are diffed
against the last-acked framebuffer using CUP-based positioning.
Fixes character doubling during fast typing.
Wire-compatible with C mosh. AES-128-OCB3, SSP transport, fragmentation,
protobuf state sync, latch extension fields.
Includes ServeRW bridge mode for embedding.
realrasengan