Reflects the async-trait addition to nodedb-cluster and advances all
workspace member versions from 0.0.2 to 0.0.3.

Introduce the dissemination module (DisseminationQueue, PendingUpdate,
apply_and_disseminate) that carries membership deltas as piggyback
payloads on every outgoing probe datagram.

Each outgoing Ping, PingReq, and Ack now attaches up to max_piggyback
rumours selected from the queue. A rumour is retired after it has been
forwarded ceil(lambda * log2(n+1)) times, matching the bound from the
SWIM:GREED paper (Das et al. §4.3) that guarantees with high probability
every live member receives each delta.

Inbound piggyback is ingested before dispatching the message so that a
self-refutation incarnation bump is reflected in the outgoing Ack of the
same round-trip. SwimConfig gains max_piggyback and fanout_lambda fields
with validation; ProbeRound and DetectorRunner are wired to pass them
through. Integration tests cover cross-node delta propagation and
self-refutation via piggyback.

Extract `InMemoryTransport` into its own file and add a real
`UdpTransport` alongside it, with the shared `Transport` trait and
`TransportFabric` living in a `transport/mod.rs` hub.

The old monolithic `transport.rs` held the trait, the in-memory
fabric, and placeholder comments for UDP; keeping them together made
the file grow as soon as UDP landed. The new layout mirrors the
established one-concern-per-file rule:
  transport/mod.rs      — re-exports + shared trait
  transport/in_memory.rs — test fabric (mpsc channels, drop injection)
  transport/udp.rs       — production UDP transport (tokio UdpSocket)

`UdpTransport` is now re-exported from the `swim` public API so cluster
startup and integration tests can construct one without reaching into
internal modules.

…g field

Callers previously had to wire up MembershipList, DisseminationQueue,
FailureDetector, and the run-loop task themselves — a four-step
sequence with easy ordering mistakes. `spawn_swim` collapses this into
one call: it seeds the membership list from the provided address list,
validates config, starts the detector task, and returns a `SwimHandle`
that owns shutdown plumbing and exposes the shared membership and
dissemination queue.

`BootstrapConfig` gains `swim_udp_addr: Option<SocketAddr>` so
operators can opt into SWIM by supplying a bind address. `None` keeps
the existing behaviour (cluster boots without SWIM; membership is
observed only through Raft). All existing callsites are updated with
`swim_udp_addr: None`.

`SwimHandle` and `spawn_swim` are re-exported from `nodedb_cluster` so
dependent crates do not need to reach into swim internals.

Adds a real-UDP integration test (`swim_udp_convergence`) that boots
three SWIM nodes on ephemeral loopback ports and asserts they converge
to a full Alive view before a member is shut down and the remainder
observe it as Suspect/Dead.