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/download.go

@@ -21,7 +21,7 @@ const (
 	downloadTimeout = time.Minute
 
 	// maxDownloadSize is the size of the largest list file accepted, in
-	// bytes. The largest list, FireHOL abusers, is about 2.5 MiB.
+	// bytes. The largest list, iCloud Private Relay, is about 12 MiB.
 	maxDownloadSize = 200 << 20
 )