common.TmuxInvocationObserver

This is the seam for logs, traces and metrics: a caller that wants to see what the library is doing has otherwise to supply a whole engine. It runs after the invocation is decided and cannot change its outcome — anything it throws is swallowed, because a command must not fail on account of the code watching it.

Returning a promise is allowed and never awaited: the invocation has already been decided, so waiting would only delay it. A rejection is swallowed on the same grounds as a throw, rather than left to the host's unhandled rejection policy.