Implement RenderForm/ParseForm for the zonemasterProvider under the
standalone build tag, enabling browser-based interactive use. Also add
a `make test` target that builds with the same tag.
The binary doubles as its own healthcheck client via the SDK's
-healthcheck flag, so the probe works in the scratch image
(no shell, no curl, no wget).
Follow the checker-sdk-go interface consolidation: reporter methods
now take sdk.ReportContext and read the payload via ctx.Data() instead
of the raw json.RawMessage parameter. Backed by the same underlying
logic.
