Step 4.1: /stream.sdp from the live config and PTP GM

- aes67_sdp_sap: aes67_sdp_build() builds the AES67 SDP (RFC 4566 +
  RFC 7273) with the same lines and order as the web UI preview;
  GET /stream.sdp serves it as application/sdp.
- aes67_ptp: public aes67_ptp_gm_id(), aes67_ptp_locked(),
  aes67_ptp_now_ns(). Clock IDs are now uppercase (RFC 7273 style), also
  in status.ptp, which the UI copies into ts-refclk.
- aes67_net: aes67_net_netif() returns the AES67 interface.
- Verified on board: /stream.sdp byte-identical to the UI's sdp() for
  the same config/status; mono, L16, ptime 0.333, ttl 8, clk_offset
  4294967295 and session_ver all follow the config.

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

components/aes67_net/aes67_net.c

@@ -183,3 +183,8 @@ esp_err_t aes67_net_init(esp_eth_handle_t eth)
     status_register(net_status);
     return esp_eth_start(eth);
 }
+
+esp_netif_t *aes67_net_netif(void)
+{
+    return s_netif;
+}

components/aes67_net/include/aes67_net.h

@@ -4,6 +4,10 @@
 
 #include "esp_err.h"
 #include "esp_eth_driver.h"
+#include "esp_netif.h"
 
 // Attach a netif (DHCP client) to the Ethernet driver and start it. Logs link and IP events.
 esp_err_t aes67_net_init(esp_eth_handle_t eth);
+
+// The AES67 interface (untagged side once the VLAN split exists). NULL before aes67_net_init().
+esp_netif_t *aes67_net_netif(void);