Step 6.2: role hand-over, session_ver bump on GM change

- aes67_cfg: cfg_set_number(group, key, value, apply) for firmware-side
  changes, stored in NVS like a POST; apply can be skipped.
- aes67_sdp_sap: once a second, a change of the PTP GM (ts-refclk) bumps
  aes67.session_ver without re-applying the aes67 group (no stream
  restart); SAP then re-announces. Runs whether or not SAP is on.
- Verified with ptp4l as a normal clock (p1 128): board auto (p1 250)
  steps down to SLAVE (session_ver 1->2); role master p1 100 via API ->
  board MASTER at once (->3); back to auto p1 250 -> ptp4l takes over
  within ~1 s, board SLAVE again (->4).

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

components/aes67_sdp_sap/aes67_sdp_sap.c

@@ -164,6 +164,26 @@ static bool sap_wanted(uint8_t *ttl)
     return on;
 }
 
+// When the PTP GM changes, the SDP's ts-refclk changes: bump the session version (RFC 4566 o=)
+// so receivers see a new description. Runs whether or not SAP is on (manual SDP users too).
+static void check_gm_change(void)
+{
+    static char last[24];
+    char gm[24];
+    if (!aes67_ptp_gm_id(gm) || strcmp(gm, last) == 0) {
+        return;
+    }
+    if (last[0]) {
+        cJSON *a = cfg_get("aes67");
+        double ver = cJSON_GetObjectItemCaseSensitive(a, "session_ver")->valuedouble + 1;
+        cJSON_Delete(a);
+        // Not applied: re-applying the aes67 group would restart the stream.
+        esp_err_t err = cfg_set_number("aes67", "session_ver", ver, false);
+        ESP_LOGI(TAG, "PTP TimeTransmitter %s -> %s: session_ver %.0f (%s)", last, gm, ver, esp_err_to_name(err));
+    }
+    strcpy(last, gm);
+}
+
 static void sap_task(void *arg)
 {
     static char sdp[SDP_MAX];
@@ -172,6 +192,7 @@ static void sap_task(void *arg)
 
     while (1) {
         vTaskDelay(pdMS_TO_TICKS(1000));
+        check_gm_change();
         esp_netif_ip_info_t ip;
         esp_netif_t *netif = aes67_net_netif();
         bool have_ip = netif && esp_netif_get_ip_info(netif, &ip) == ESP_OK && ip.ip.addr;

components/aes67_web/cfg.c

@@ -232,6 +232,37 @@ cJSON *cfg_get(const char *group)
     return copy;
 }
 
+esp_err_t cfg_set_number(const char *group, const char *key, double value, bool apply)
+{
+    if (!s_lock) {
+        return ESP_ERR_INVALID_STATE;
+    }
+    xSemaphoreTake(s_lock, portMAX_DELAY);
+    cfg_group_t *g = find(group);
+    cJSON *cur = g ? cJSON_GetObjectItemCaseSensitive(g->values, key) : NULL;
+    esp_err_t err = ESP_OK;
+    if (!cur || !cJSON_IsNumber(cur)) {
+        err = ESP_ERR_NOT_FOUND;
+    } else {
+        cJSON_SetNumberValue(cur, value);
+        nvs_handle_t h;
+        if (s_nvs_ok && (err = nvs_open(NVS_NAMESPACE, NVS_READWRITE, &h)) == ESP_OK) {
+            err = store(h, g);
+            if (err == ESP_OK) {
+                err = nvs_commit(h);
+            }
+            nvs_close(h);
+        }
+    }
+    cJSON *copy = err == ESP_OK && apply && g->apply ? cJSON_Duplicate(g->values, true) : NULL;
+    xSemaphoreGive(s_lock);
+    if (copy) {
+        g->apply(copy);
+        cJSON_Delete(copy);
+    }
+    return err;
+}
+
 /* ----- HTTP ----- */
 
 static esp_err_t config_get(httpd_req_t *req)

components/aes67_web/include/aes67_cfg.h

@@ -20,6 +20,9 @@ esp_err_t cfg_register(const char *group, const char *defaults_json,
                        cfg_validate_cb_t validate, cfg_apply_cb_t apply);
 // Copy of a group's current values; caller frees with cJSON_Delete. NULL if not registered.
 cJSON *cfg_get(const char *group);
+// Firmware-side change of one number (e.g. aes67.session_ver): stored like a POST, no validation.
+// apply = false skips the group's apply callback (e.g. to avoid restarting the stream).
+esp_err_t cfg_set_number(const char *group, const char *key, double value, bool apply);
 
 // Validation helpers for validate callbacks. Each returns false and fills err on failure.
 bool cfg_check_num(const cJSON *g, const char *key, double min, double max, char *err, size_t n);