Add the iCloud Private Relay list to the IP lookup, with is_relay (closes #237)

`icloud_private_relay` is Apple's list of Private Relay networks, downloaded once a day. Every result gains `flags.is_relay`.

`parseProtonVPN` becomes `parseLinesNotEmpty`, shared with the new parser, which reads only each line's first column.

Judgement call: a line with no comma fails the parse, so a plain list of networks at Apple's URL cannot replace a good copy.

Model: opus-5-5

internal/iplist/parse.go

@@ -25,6 +25,10 @@ const (
 	// dshieldFieldSeparator separates the fields of a DShield line.
 	dshieldFieldSeparator = "\t"
 
+	// iCloudPrivateRelayFieldSeparator separates the fields of a line of
+	// Apple's iCloud Private Relay file.
+	iCloudPrivateRelayFieldSeparator = ","
+
 	// spamhausMetadataType is the type of the line that ends a Spamhaus
 	// file. That line holds the file's metadata and no network.
 	spamhausMetadataType = "metadata"
@@ -59,7 +63,9 @@ func parse(r io.Reader, format Format) (*netipx.IPSet, error) {
 	case FormatIVPNJSON:
 		return parseIVPN(r)
 	case FormatProtonVPN:
-		return parseProtonVPN(r)
+		return parseLinesNotEmpty(r, addAddressOrNetwork)
+	case FormatICloudPrivateRelayCSV:
+		return parseLinesNotEmpty(r, addICloudPrivateRelayNetwork)
 	default:
 		return nil, errUnknownFormat
 	}
@@ -331,11 +337,12 @@ func parseIVPN(r io.Reader) (*netipx.IPSet, error) {
 	return addressSet(texts)
 }
 
-// parseProtonVPN returns the addresses or networks in a ProtonVPN-IPs list. A
-// file with no addresses or networks fails, because the service always has
-// servers.
-func parseProtonVPN(r io.Reader) (*netipx.IPSet, error) {
-	addresses, err := parseLines(r, addAddressOrNetwork)
+// parseLinesNotEmpty is parseLines for a list that always holds addresses, so
+// a file with none fails.
+func parseLinesNotEmpty(
+	r io.Reader, addLine func(*netipx.IPSetBuilder, string) error,
+) (*netipx.IPSet, error) {
+	addresses, err := parseLines(r, addLine)
 	if err != nil {
 		return nil, err
 	}
@@ -347,6 +354,24 @@ func parseProtonVPN(r io.Reader) (*netipx.IPSet, error) {
 	return addresses, nil
 }
 
+// addICloudPrivateRelayNetwork adds the network at the start of a line of
+// Apple's iCloud Private Relay file to builder.
+func addICloudPrivateRelayNetwork(builder *netipx.IPSetBuilder, line string) error {
+	field, _, found := strings.Cut(line, iCloudPrivateRelayFieldSeparator)
+	if !found {
+		return errNoNetwork
+	}
+
+	network, err := netip.ParsePrefix(field)
+	if err != nil {
+		return err
+	}
+
+	builder.AddPrefix(network)
+
+	return nil
+}
+
 // readJSON reads r to its end and decodes it, as one JSON document, into v.
 func readJSON(r io.Reader, v any) error {
 	data, err := io.ReadAll(r)