Step 2a (2/2): LLDP transmit, switch shows name and IP

- aes67_net/lldp.c: IEEE 802.1AB LLDPDU sent as a raw frame with
  esp_eth_transmit every 30 s (TTL 120 s) and immediately on a new IP
  or hostname change. TLVs: chassis/port ID (MAC), port description,
  system name (net.hostname), system description (project + version),
  capabilities station-only, IPv4 management address.
- Docs: LLDP in the core Network section; step 2a ticked.
- Verified on an HP 2530-8G-PoE+ (show lldp info remote-device 2): all
  fields shown, management address 192.168.192.244.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

components/aes67_net/lldp.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include "esp_eth_driver.h"
+#include "esp_netif.h"
+
+// Send an LLDPDU every 30 s (TTL 120 s).
+esp_err_t lldp_start(esp_eth_handle_t eth, esp_netif_t *netif);
+// Send now (e.g. after an IP or hostname change) and log the next one.
+void lldp_changed(void);
+void lldp_send(void);