doctor: print which nameservers the reality check actually used

Debug line to confirm whether the systemd-resolved D-Bus path (real
per-link servers) or the resolv.conf fallback is engaged for a given
query — needed to disambiguate cases where both paths happen to agree
on a nonexistent domain.

Author: casablanque-code

gai/src/doctor.rs

@@ -33,6 +33,11 @@ pub fn run(name: &str) -> anyhow::Result<()> {
         reality::check(name, effective_servers).ok()
     };
 
+    println!(
+        "  (reality check via {:?}, systemd-resolved stub: {})\n",
+        effective_servers, resolv.is_systemd_stub
+    );
+
     println!("RESOLUTION PATH (simulated):");
     for (i, step) in outcome.steps.iter().enumerate() {
         println!("  {}. [{:?}] {:?}", i + 1, step.source, step.result);