perf: name-first geo detection to avoid serial DNS in collection rename

DetectGeoWithServer was doing GeoIP-first (DNS lookup per node) then falling back to name regex. For a 148-node collection with rename enabled, this meant 148 serial DNS queries in the rename pass alone (28s on slow DNS).

Reverse the priority: try name-based regex first (instant, no I/O), only fall back to GeoIP+DNS when the node name has no recognizable country keywords. Most proxy names already contain country/city info.

Also add in-memory DNS cache in geoip.LookupHost so repeated lookups for the same hostname don't re-resolve.

Collection preview with rename: 30s -> 3s.

internal/util/flag.go

@@ -164,10 +164,15 @@ func DetectGeo(name string) GeoInfo {
 	return detectGeoFromName(name)
 }
 
-// DetectGeoWithServer returns geographic info by looking up the proxy's
-// server address via GeoIP first, falling back to name-based detection.
+// DetectGeoWithServer returns geographic info by first trying name-based
+// regex matching (fast, no I/O), falling back to GeoIP lookup on the server
+// address only when the name doesn't contain recognizable geo keywords.
 func DetectGeoWithServer(name, server string) GeoInfo {
-	// Try GeoIP lookup first
+	// Try name-based detection first (fast, no I/O)
+	if geo := detectGeoFromName(name); geo.CountryName != "" {
+		return geo
+	}
+	// Fallback to GeoIP lookup (slow, involves DNS)
 	if server != "" {
 		host := server
 		// Strip port if present
@@ -184,8 +189,7 @@ func DetectGeoWithServer(name, server string) GeoInfo {
 			}
 		}
 	}
-	// Fallback to name-based detection
-	return detectGeoFromName(name)
+	return GeoInfo{}
 }
 
 // detectGeoFromName does name-based regex matching for geo detection.