Initial commit

CalDAV and CardDAV checkers sharing a single Go module. Discovery follows
RFC 6764 (/.well-known + SRV/TXT), authenticated probes cover principal,
home-set, collections and a minimal REPORT query on top of go-webdav.
Common shape in internal/dav/; CalDAV adds a scheduling rule.

Surfaces its context URL (and each secure-SRV target) as TLS endpoints via
the EndpointDiscoverer interface, so the dedicated TLS checker can pick
them up without re-parsing observations.

HTML report foregrounds common misconfigs (well-known returning 200,
missing SRV, plaintext-only SRV, missing DAV capability, skipped auth
phase) as action-item callouts before the full phase breakdown.

caldav/provider.go

@@ -0,0 +1,25 @@
+package caldav
+
+import (
+	"net/http"
+
+	"git.happydns.org/checker-dav/internal/dav"
+	sdk "git.happydns.org/checker-sdk-go/checker"
+)
+
+// Provider's return value also satisfies CheckerDefinitionProvider,
+// CheckerHTMLReporter, and EndpointDiscoverer; the SDK server probes for
+// those at runtime.
+func Provider() sdk.ObservationProvider {
+	return &caldavProvider{}
+}
+
+type caldavProvider struct{}
+
+func (p *caldavProvider) Key() sdk.ObservationKey { return ObservationKey }
+
+func (p *caldavProvider) RenderForm() []sdk.CheckerOptionField { return dav.InteractiveForm() }
+
+func (p *caldavProvider) ParseForm(r *http.Request) (sdk.CheckerOptions, error) {
+	return dav.ParseInteractiveForm(r)
+}