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/iplist.go

@@ -56,6 +56,11 @@ const (
 	// that start with #. Unlike FormatLines, a file with no addresses or
 	// networks fails.
 	FormatProtonVPN
+
+	// FormatICloudPrivateRelayCSV is Apple's CSV file of iCloud Private Relay
+	// networks. Each line starts with a network, followed by its country,
+	// region and city.
+	FormatICloudPrivateRelayCSV
 )
 
 // List is one public IP list.