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>

main/main.c

@@ -3,6 +3,7 @@
 #include "aes67_net.h"
 #include "aes67_ota.h"
 #include "aes67_ptp.h"
+#include "aes67_sdp_sap.h"
 #include "aes67_syslog.h"
 #include "aes67_tx.h"
 #include "aes67_web.h"
@@ -36,5 +37,6 @@ void app_main(void)
     ESP_ERROR_CHECK(aes67_syslog_init());
     project_cfg_register();
     ESP_ERROR_CHECK(aes67_ota_init());
+    ESP_ERROR_CHECK(aes67_sdp_sap_init());
     ESP_ERROR_CHECK(aes67_web_start());
 }