engine.TmuxEngine

Two obligations beyond returning bytes, and both are what the layers above assume rather than check:

Every request is one tmux invocation and carries its ordered command list. Keeping that structure at the seam prevents an engine from splitting a snapshot into several clients or guessing where global flags end. Environment, stdin, signal, and timeout apply once to the whole invocation.

daemonGuard, when a request carries one, must reach tmux — or the engine must be bound to one daemon for its lifetime, the way a control connection is. Ignoring it on a reconnecting engine means a handle read before a restart addressing whatever now holds its id. guardRequest is what the built-in engine calls to honour it, published so an implementer does not reproduce the wrapper, its else branch, and the stderr that tells refusal from failure.