transport: derive Serialize for EthernetStatsSnapshot

EthernetStatsSnapshot derived only Clone/Debug/Default, unlike every
other transport's *StatsSnapshot which also derives Serialize. That gap
forced a hand-rolled serde_json::json!{} arm in TransportHandle::
transport_stats() that re-listed all ten fields by hand. Add the
Serialize derive and collapse the arm to the same one-line
serde_json::to_value(...) form the other transports use. The emitted
JSON keys and values are unchanged.

src/transport/ethernet/stats.rs

@@ -2,6 +2,8 @@
 
 use portable_atomic::{AtomicU64, Ordering};
 
+use serde::Serialize;
+
 /// Statistics for an Ethernet transport instance.
 ///
 /// Uses atomic counters for lock-free updates from the receive loop
@@ -92,7 +94,7 @@ impl Default for EthernetStats {
 }
 
 /// Point-in-time snapshot of Ethernet stats (non-atomic, copyable).
-#[derive(Clone, Debug, Default)]
+#[derive(Clone, Debug, Default, Serialize)]
 pub struct EthernetStatsSnapshot {
     pub frames_sent: u64,
     pub frames_recv: u64,

src/transport/mod.rs

@@ -1022,19 +1022,7 @@ impl TransportHandle {
             }
             #[cfg(unix)]
             TransportHandle::Ethernet(t) => {
-                let snap = t.stats().snapshot();
-                serde_json::json!({
-                    "frames_sent": snap.frames_sent,
-                    "frames_recv": snap.frames_recv,
-                    "bytes_sent": snap.bytes_sent,
-                    "bytes_recv": snap.bytes_recv,
-                    "send_errors": snap.send_errors,
-                    "recv_errors": snap.recv_errors,
-                    "beacons_sent": snap.beacons_sent,
-                    "beacons_recv": snap.beacons_recv,
-                    "frames_too_short": snap.frames_too_short,
-                    "frames_too_long": snap.frames_too_long,
-                })
+                serde_json::to_value(t.stats().snapshot()).unwrap_or_default()
             }
             TransportHandle::Tcp(t) => {
                 serde_json::to_value(t.stats().snapshot()).unwrap_or_default()