Add typed serial settings operations

Route bounded admin mutations through the existing administration
dispatcher,
covering apply, lifecycle, persistence, authorization, and result
tracking.
Add the browser controls, automatic result refresh, regression coverage,
and
phase documentation.

src/web_httpd_adapter.h

@@ -16,3 +16,5 @@ esp_err_t web_httpd_upgrade(httpd_req_t *request,
 /* Serialized server startup only, exact-match ordinary GET, URI <= 127 bytes.
  * Stage both allocations before publication; HTTPD owns/frees them on success. */
 esp_err_t web_httpd_register_optional_get(httpd_handle_t server, const httpd_uri_t *uri);
+/* Same staged startup ownership for ordinary exact GET or POST. */
+esp_err_t web_httpd_register_optional(httpd_handle_t server, const httpd_uri_t *uri);