Add the CalDAV client and a CLI to drive it

Discovery is the three PROPFINDs RFC 4791 describes rather than a walk
through likely URLs. Queries use time-range, which v1 never did -- it
fetched whole calendars and filtered in the browser on every view change.
Every write states a precondition, so a stale ETag produces a Conflict a
caller can act on instead of silently destroying somebody's edit.

XML goes through quick-xml with namespace resolution. v1 matched prefixes
with six regexes tried in sequence and recompiled inside the loop; there
is a fixture here that is the same document under different prefixes, and
it parses identically.

Protocol parsing is split from transport so it can be tested against
responses recorded from a real Baikal -- including the second propstat
carrying 404s, which is what makes "this calendar has no colour" different
from "this calendar has an empty colour".

Live tests run against a real server, never a mock. tests/baikal/run.sh
starts a container, walks Baikal's install wizard, and runs them; each
test builds and destroys its own collection, so pointing it at a real
server touches nothing that was already there. They cover discovery,
round-trip, stale-ETag conflict, duplicate create, delete, time-range
filtering, a series returned whole with its override, and writing every
synthetic golden fixture to the server and reading it back.

libdav was evaluated first, as planned. Not adopted: its HttpClient trait
is defined over hyper::body::Incoming, so using it means replacing reqwest
everywhere, plus a DNS resolver for service discovery we do not do and a
second XML parser. Its precondition design is where Precondition's shape
comes from. Reasons are recorded in the crate docs.

Cargo.toml

@@ -58,6 +58,9 @@ quick-xml = "0.42"
 leptos = "0.8"
 leptos_router = "0.8"
 
+# CLI
+clap = { version = "4", features = ["derive", "env"] }
+
 # Dev
 pretty_assertions = "1"