Fluxion::Executor::Probe

Decides whether an item is already present, without changing anything.

Probes are what make a rerun cheap and a plan honest. They must be read-only, idempotent, and bounded: status, diff, and plan --skip-already-installed all run every probe in a profile, so one that hangs makes the whole command useless.

An unanswerable probe reports Unknown rather than NotInstalled. The difference matters: absence of evidence would silently reinstall things, and the user is told which items Fluxion could not check.