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_sdp_sap/include/aes67_sdp_sap.h

@@ -2,6 +2,11 @@
 // Core component: must not depend on main/ (project code).
 #pragma once
 
+#include <stddef.h>
+
 #include "esp_err.h"
 
+// Registers GET /stream.sdp. (SAP: step 5.)
 esp_err_t aes67_sdp_sap_init(void);
+// Build the current SDP (RFC 4566 + RFC 7273, AES67). Returns the length, or 0 if it didn't fit.
+size_t aes67_sdp_build(char *buf, size_t size);