The reverse-DNS observation (zone location, authoritative NS, the PTR RRset,
and the forward-confirm of the effective target) is determined entirely by the
reverse-arpa owner name — i.e. the IP being asked about — never by which
forward domain triggered the check. Implement sdk.ObservationSharer so the host
runs the PTR + FCrDNS lookups once and serves every target that interrogates
the same reverse name, instead of re-querying per record.
The share key derives from the owner name and folds in the declared target and
TTL: these are part of the observation and can change it — when no PTR is
published the effective target (and therefore its forward-confirm) falls back
to the declared value. ShareKey stays a pure function of opts (no network) per
the contract. An empty owner yields "" so the host falls back to per-target
caching.
