Add Ethernet transport with beacon discovery

Implement raw Ethernet transport using AF_PACKET SOCK_DGRAM on Linux
with EtherType 0x88B5 (IEEE experimental range) and 1-byte frame type
prefix (0x00=data, 0x01=beacon).

Transport implementation:
- EthernetConfig with interface, ethertype, MTU, buffer sizes, and
  four independent discovery knobs (discovery, announce, auto_connect,
  accept_connections)
- PacketSocket/AsyncPacketSocket wrappers with ioctl helpers for
  interface index, MAC address, and MTU queries
- EthernetTransport with Transport trait impl, async start/stop/send,
  receive loop dispatching data frames and discovery beacons
- Discovery beacons (34 bytes: type + version + x-only pubkey) with
  DiscoveryBuffer for peer accumulation and dedup
- Atomic statistics counters (frames, bytes, errors, beacons)
- Platform-gated with #[cfg(target_os = "linux")]

Transport-layer discovery integration:
- Promote auto_connect() and accept_connections() to Transport trait
  with default implementations and TransportHandle dispatch
- Extract initiate_connection() so both static peer config and
  discovery auto-connect share the same handshake initiation path
- Add poll_transport_discovery() to the tick handler to drain
  discovery buffers and auto-connect to discovered peers
- Enforce accept_connections() in handle_msg1() — transports with
  accept_connections=false silently drop inbound handshakes

Node integration:
- create_transports() handles Ethernet named instances
- resolve_ethernet_addr() parses "interface/mac" address format
- transport_mtu() generalized for multi-transport operation

Test harness:
- VethPair RAII struct for veth pair lifecycle management
- Three #[ignore] integration tests requiring root/CAP_NET_RAW:
  two-node handshake, data exchange, mixed transport coexistence
- Chaos harness: transport-aware topology model, VethManager for
  veth pairs between Docker containers, Ethernet-aware config gen,
  netem split (HTB+u32 for UDP, root netem for veth), transport-aware
  link flaps and node churn with veth re-setup
- Container entrypoint waits for configured Ethernet interfaces
  before starting FIPS (handles veth creation timing)
- New scenarios: ethernet-only (4-node ring), ethernet-mesh (6-node
  mixed UDP+Ethernet with netem and link flaps)

Documentation:
- fips-transport-layer.md: Ethernet section, beacon discovery, WiFi
  compatibility, updated discovery state, trait surface additions,
  implementation status table
- fips-configuration.md: Ethernet parameter table, named instances,
  peer address format, mixed UDP+Ethernet example, complete reference
- fips-wire-formats.md: Ethernet frame type prefix note

src/node/tree.rs

@@ -302,10 +302,10 @@ impl Node {
         let now = std::time::Instant::now();
         let interval = std::time::Duration::from_secs(interval_secs);
 
-        if let Some(last) = self.last_parent_reeval {
-            if now.duration_since(last) < interval {
-                return;
-            }
+        if let Some(last) = self.last_parent_reeval
+            && now.duration_since(last) < interval
+        {
+            return;
         }
 
         self.last_parent_reeval = Some(now);