The CalDAV and CardDAV clients found their payloads by splitting the
response body on the literal strings `<c:calendar-data>` and
`<card:address-data>`. That is only correct for servers that happen to
pick the same prefixes Nuncio does. A compliant server that binds the
namespaces to `D:`/`CAL:`, or declares `DAV:` as the default namespace,
produced zero events or zero contacts and reported success -- the user
saw an empty calendar with nothing in the logs. An element whose name
merely contains the searched one (`<c:calendar-data-summary>`) was read
as if it were the payload, and a property returned under a `404`
propstat was treated as a value.
Add `nuncio_core::dav`, a namespace-aware `multistatus` walker built on
quick-xml (already a workspace dependency, previously declared but
unused) and shared by both clients, since RFC 4791 and RFC 6352 return
the same document shape. Elements are matched on their resolved
`{namespace}local-name`; `<propstat>` status is honoured; and a body
that cannot be traversed is now a `MalformedResponse` error rather than
an empty result indistinguishable from an empty collection. Responses
that carry no readable value are counted and logged at `warn` with the
href and reason, so an omission is visible instead of silent.
Refs #257
James Maes