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.

internal/dav/util.go

@@ -0,0 +1,7 @@
+package dav
+
+import "net/url"
+
+// parseURL is an alias so principal.go can use net/url without importing it
+// directly (keeping imports tidy across files).
+func parseURL(raw string) (*url.URL, error) { return url.Parse(raw) }