v0.9.6

Headline: a built-in mock ONVIF device so downstream crates can unit-test
client code without a real camera — every vendor's ONVIF differs and depending
on a physical IP camera in tests is painful. Also rolls in the session-level
push subscribe and vendor-tolerant OSD parsing.

Added

• Built-in mock ONVIF device — oxvif::mock (opt-in, behind features).
  Stateful (Set persists, Get reflects it) and covers every operation oxvif
  implements; state is in-memory (the library never writes to disk — opt into
  persistence via MockState::set_on_change).
  • mock feature → MockTransport, an in-process Transport (no sockets, no
    axum) — the fast unit-test path:
    OnvifClient::new("http://mock").with_transport(Arc::new(MockTransport::new())).
  • mock-server feature → MockServer, a real axum HTTP server on an
    ephemeral port (MockServer::start().await), shutting down on drop — for
    cross-process / non-Rust clients.
  • Both default to no auth (call .with_auth() / .enforce_auth(true) to
    exercise WS-Security) and support inject_fault(...) for error-path tests.
  • axum / serde are optional deps enabled only by these features — the
    default build is unchanged and axum-free.
  • The examples/mock_server binary is now a thin wrapper over MockServer
    with TOML file persistence (--features mock-server); the mock engine moved
    from examples/ into src/mock/. New tests/mock_workflow.rs drives one
    command from every service against a real MockServer.
• OnvifSession::subscribe — delegates the WS-BaseNotification push
  subscription that was previously only on OnvifClient.
• OsdOptions::max_per_text_type. New HashMap<String, u32>
  exposing the per-text-type quotas (Plain, Date, Time,
  DateAndTime) some cameras advertise via XML attributes on
  <MaximumNumberOfOSDs> (Genetec, recent Hikvision). Lets clients
  pre-validate CreateOSD calls against per-type limits instead of
  parsing opaque ter:InvalidArgs fault strings after the fact.
  Populated only when fetched via OnvifSession::get_osd_options
  — OnvifClient::get_osd_options leaves it empty (spec-strict).
• OnvifSession::get_osd_options now layers vendor-extension
  parsing on top of the spec-strict OnvifClient result. Two
  real-world shapes handled:
  • <MaximumNumberOfOSDs Total="8" Plain="7" DateAndTime="1" .../>
    — count from Total attribute when element body is empty, plus
    per-type quotas from named attributes.
  • <PositionOption>UpperLeft</PositionOption> flat siblings, when
    the textbook nested-<Type> shape produces nothing.