test: run node-level mesh tests over an in-process loopback transport

Add a Loopback variant to TransportHandle backed by an unbounded
in-process channel and a shared address-to-receiver registry, so
node-level multi-node tests deliver packets directly between nodes
instead of over real localhost UDP sockets. This removes the kernel
UDP receive-buffer overflow that dropped handshake packets when many
tests ran in parallel under CPU contention, and lets the large-network
convergence tests run reliably in the default suite again (their
parallel-load ignore markers are removed).

The new transport and its enum variant are cfg(test)-gated, so the
daemon build is unaffected.

src/node/tests/mod.rs

@@ -1,6 +1,6 @@
 use super::*;
 use crate::PeerIdentity;
-use crate::transport::{LinkDirection, TransportAddr, packet_channel};
+use crate::transport::{LinkDirection, ReceivedPacket, TransportAddr, packet_channel};
 use crate::utils::index::SessionIndex;
 use std::time::Duration;