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/ble.rs

@@ -63,7 +63,7 @@ async fn make_test_node_ble(node_num: u8) -> TestNode {
     TestNode {
         node,
         transport_id,
-        packet_rx,
+        packet_rx: spanning_tree::bridge_to_unbounded(packet_rx),
         addr: ta,
     }
 }
@@ -303,7 +303,7 @@ async fn test_ble_discovery() {
     let mut nodes = vec![TestNode {
         node,
         transport_id,
-        packet_rx,
+        packet_rx: spanning_tree::bridge_to_unbounded(packet_rx),
         addr: ta,
     }];
     cleanup_nodes(&mut nodes).await;