PTP: drop the "Preferred TimeTransmitter" role

It behaved exactly like auto (clock class 248); the only difference was a
priority1 suggestion in the page. Roles are now slave|auto; to prefer this
device, use auto with a low priority1. A stored "master" is converted to
auto at boot (new cfg_set_string, like cfg_set_number).

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

components/aes67_ptp/aes67_ptp.c

@@ -1,7 +1,10 @@
 #include "aes67_ptp.h"
 
+#include <string.h>
+
 #include "aes67_cfg.h"
 #include "aes67_web.h"
+#include "esp_log.h"
 #include "ptp_clock.h"
 #include "ptp_hw.h"
 
@@ -14,7 +17,7 @@ static const char PTP_DEFAULTS[] =
 static bool ptp_validate(const cJSON *g, char *err, size_t n)
 {
     static const char *const modes[] = { "multicast", "hybrid", NULL };
-    static const char *const roles[] = { "slave", "auto", "master", NULL };
+    static const char *const roles[] = { "slave", "auto", NULL };
     return cfg_check_enum(g, "mode", modes, err, n) &&
            cfg_check_enum(g, "role", roles, err, n) &&
            cfg_check_int(g, "domain", 0, 127, err, n) &&
@@ -34,7 +37,20 @@ static void ptp_apply(const cJSON *g)
 
 esp_err_t aes67_ptp_init(void)
 {
-    return cfg_register("ptp", PTP_DEFAULTS, ptp_validate, ptp_apply);
+    esp_err_t err = cfg_register("ptp", PTP_DEFAULTS, ptp_validate, ptp_apply);
+    if (err != ESP_OK) {
+        return err;
+    }
+    // Role "master" was dropped (it behaved like auto; a low priority1 does the same). Stored values
+    // aren't re-validated, so convert it here.
+    cJSON *c = cfg_get("ptp");
+    bool master = !strcmp(cJSON_GetObjectItemCaseSensitive(c, "role")->valuestring, "master");
+    cJSON_Delete(c);
+    if (master) {
+        ESP_LOGW("ptp", "stored role \"master\" is no longer offered: now \"auto\" (same behaviour)");
+        cfg_set_string("ptp", "role", "auto", false);
+    }
+    return ESP_OK;
 }
 
 esp_err_t aes67_ptp_start(esp_eth_handle_t eth)