transport/ethernet: rename config discovery flag to listen

Rename the Ethernet per-interface config flag from discovery to listen,
so the receive/transmit toggle pair reads as the symmetric announce
(transmit) / listen (receive) neighbor-beacon vocabulary. The old
discovery: key is still accepted via a serde alias, so deployed configs
load unchanged; to_yaml re-emits it under the canonical listen: name.
Marked deprecated for removal at the v2 cutover.

Updates the config field + accessor, the transport listen_enabled local,
the one struct-literal test consumer, the chaos sim config generator,
packaged fips.yaml examples, and the classified operator-facing docs
(ethernet neighbor-beacon subsystem prose; the generic Transport
discovery capability prose is left unchanged). Adds a compat parse test
asserting the legacy alias, the new key, and that deny_unknown_fields
still rejects unknown keys. Behavior-neutral.

src/node/tests/ethernet.rs

@@ -83,7 +83,7 @@ async fn make_test_node_ethernet(interface: &str) -> TestNode {
 
     let config = EthernetConfig {
         interface: interface.to_string(),
-        discovery: Some(false),
+        listen: Some(false),
         announce: Some(false),
         accept_connections: Some(true),
         ..Default::default()