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/include/aes67_ptp.h

@@ -2,6 +2,9 @@
 // Core component: must not depend on main/ (project code).
 #pragma once
 
+#include <stdbool.h>
+#include <stdint.h>
+
 #include "esp_err.h"
 #include "esp_eth_driver.h"
 
@@ -9,3 +12,10 @@
 esp_err_t aes67_ptp_init(void);
 // Start the EMAC PTP clock and the PTP task on this Ethernet interface.
 esp_err_t aes67_ptp_start(esp_eth_handle_t eth);
+
+// Current GM clockIdentity as "XX-XX-XX-XX-XX-XX-XX-XX" (RFC 7273 form). false if none selected.
+bool aes67_ptp_gm_id(char out[24]);
+// Servo locked to the GM.
+bool aes67_ptp_locked(void);
+// PTP time (EMAC hardware clock) in ns.
+esp_err_t aes67_ptp_now_ns(int64_t *ns);