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/smartacme.classes.smartacme.ts

@@ -169,6 +169,14 @@ export class SmartAcme {
       process.removeListener('SIGTERM', this.boundSigtermHandler);
       this.boundSigtermHandler = null;
     }
+    // Destroy ACME HTTP transport (closes keep-alive sockets)
+    if (this.client) {
+      this.client.destroy();
+    }
+    // Destroy DNS client (kills Rust bridge child process if spawned)
+    if (this.smartdns) {
+      this.smartdns.destroy();
+    }
     if (this.certmanager && typeof (this.certmanager as any).close === 'function') {
       await (this.certmanager as any).close();
     }