fix(acme-http-client): Destroy keep-alive HTTP agents and DNS client on shutdown to allow process exit; add destroy() on AcmeHttpClient and AcmeClient, wire agents into requests, and call client/smartdns destroy during SmartAcme.stop; documentation clarifications and expanded README (error handling, examples, default retry values).

ts/acme/acme.classes.client.ts

@@ -96,4 +96,11 @@ export class AcmeClient {
   async getCertificate(order: IAcmeOrder): Promise<string> {
     return this.orderManager.getCertificate(order);
   }
+
+  /**
+   * Destroy HTTP transport to release sockets and allow process exit.
+   */
+  destroy(): void {
+    this.httpClient.destroy();
+  }
 }