Interaction & keepalive

Single/double click, the 0xBF request multiplexer, and ping round-trips.

Opcode	Name	Dir	Size	Description
0x06	Double Click	C → S	5 bytes (fixed)	The client double-clicked an entity, identified by its serial.
0x09	Single Click	C → S	5 bytes (fixed)	The client clicked an entity, identified by its serial.
0x73	Ping	C → S	2 bytes (fixed)	The client sends this periodically with a rolling sequence byte and expects the server to echo it straight back, or it eventually drops the connection.
0x73	Ping Ack	S → C	2 bytes (fixed)	Echoes the client's keep-alive sequence byte straight back.
0xBF	General Information	C → S	Variable	A multiplexed request whose meaning is chosen by a leading SubCommand (ushort).