v0.2.0 — an unresolvable threshold is Unknown, not Below

Added — NexusStatus::Unknown

A state whose threshold cannot be resolved is now reported as Unknown rather than Below, and NexusReport::unknown() exposes those states.

The threshold source is remote, so a firewalled deployment, a misconfigured location, or a mirror serving a layout the consumer does not read resolves null for every state — and reporting that as Below reads as "no obligation" when the honest answer is "cannot tell". A seller could cross every threshold in the country and see a clean board.

Hosts should surface unknown() wherever they surface triggered() and approaching(). An alert sweep that iterates only the latter two sends nothing at all on a deployment whose dataset is unreadable, which renders "I don't know" as silence — the worst possible rendering for a tax obligation.