The SDK split the HTTP server scaffolding into the new
checker-sdk-go/checker/server subpackage. Update main.go to import
server and call server.New, and isolate the interactive form code
behind the standalone build tag so plugin/builtin builds skip
net/http and html/template entirely.
Implements RenderForm/ParseForm on the provider: users can hit /check
with just a domain name; ParseForm resolves CAA records via direct DNS
queries (walking up the label tree per RFC 8659) and hands the SDK a
synthetic service payload so the standard Collect → Evaluate pipeline
runs without a happyDomain host. TLS probes are not gathered here, so
the rule reports StatusUnknown for the TLS cross-check in this mode.
