transport: give ethernet a canonical addr.rs home for MAC parsing

Moves parse_mac_string into ethernet/addr.rs and re-exports it at the
ethernet module root so the ethernet::parse_mac_string path stays
byte-identical for its one external consumer (zero churn there). Its
unit tests stay in mod.rs, calling through the re-export. Pure
relocation; no logic change.

src/transport/ethernet/addr.rs

@@ -0,0 +1,21 @@
+//! Ethernet address parsing.
+
+use crate::transport::TransportError;
+
+/// Parse a colon-separated MAC string (e.g., "aa:bb:cc:dd:ee:ff") into bytes.
+pub fn parse_mac_string(s: &str) -> Result<[u8; 6], TransportError> {
+    let parts: Vec<&str> = s.split(':').collect();
+    if parts.len() != 6 {
+        return Err(TransportError::InvalidAddress(format!(
+            "invalid MAC format: expected 6 colon-separated hex bytes, got '{}'",
+            s
+        )));
+    }
+    let mut mac = [0u8; 6];
+    for (i, part) in parts.iter().enumerate() {
+        mac[i] = u8::from_str_radix(part, 16).map_err(|_| {
+            TransportError::InvalidAddress(format!("invalid hex byte '{}' in MAC address", part))
+        })?;
+    }
+    Ok(mac)
+}

src/transport/ethernet/mod.rs

@@ -5,10 +5,13 @@
 //! Works on wired Ethernet and WiFi interfaces (kernel mac80211 abstracts
 //! 802.11 transparently on Linux).
 
+pub mod addr;
 pub mod discovery;
 pub mod io;
 pub mod stats;
 
+pub use addr::parse_mac_string;
+
 use super::{
     DiscoveredPeer, PacketTx, ReceivedPacket, Transport, TransportAddr, TransportError,
     TransportId, TransportState, TransportType,
@@ -632,24 +635,6 @@ pub fn format_mac(mac: &[u8; 6]) -> String {
     )
 }
 
-/// Parse a colon-separated MAC string (e.g., "aa:bb:cc:dd:ee:ff") into bytes.
-pub fn parse_mac_string(s: &str) -> Result<[u8; 6], TransportError> {
-    let parts: Vec<&str> = s.split(':').collect();
-    if parts.len() != 6 {
-        return Err(TransportError::InvalidAddress(format!(
-            "invalid MAC format: expected 6 colon-separated hex bytes, got '{}'",
-            s
-        )));
-    }
-    let mut mac = [0u8; 6];
-    for (i, part) in parts.iter().enumerate() {
-        mac[i] = u8::from_str_radix(part, 16).map_err(|_| {
-            TransportError::InvalidAddress(format!("invalid hex byte '{}' in MAC address", part))
-        })?;
-    }
-    Ok(mac)
-}
-
 // ============================================================================
 // Tests
 // ============================================================================