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

README.md

@@ -613,8 +613,9 @@ The ceilings add up to 3.7 GiB:
 The `icloud_private_relay` list has about 285,000 networks. Neighbouring
 networks merge, which leaves about 6,900 address ranges. Loaded, the list adds
 about 0.3 MiB to the Go heap and under 6 MiB to the process's resident memory.
-Parsing the file raises the process's peak by up to about 55 MiB for about 0.2
-seconds. That happens at startup and whenever a download brings a new version.
+Parsing the file allocates about 94 MiB in about 0.2 seconds. The next garbage
+collection frees all of it but the 0.3 MiB above. Parsing happens at startup and
+whenever a download brings a new version.
 
 The SQLite library gets its memory from glibc, the C library. glibc keeps freed
 memory in pools called arenas, and hands it back to the kernel slowly. It