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_ptp/aes67_ptp.c

@@ -45,3 +45,23 @@ esp_err_t aes67_ptp_start(esp_eth_handle_t eth)
     }
     return err;
 }
+
+bool aes67_ptp_gm_id(char out[24])
+{
+    return ptp_clock_gm_id(out);
+}
+
+bool aes67_ptp_locked(void)
+{
+    return ptp_clock_locked();
+}
+
+esp_err_t aes67_ptp_now_ns(int64_t *ns)
+{
+    eth_mac_time_t t;
+    esp_err_t err = ptp_hw_get_time(&t);
+    if (err == ESP_OK) {
+        *ns = (int64_t)t.seconds * 1000000000LL + t.nanoseconds;
+    }
+    return err;
+}