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

Adds Apple's iCloud Private Relay egress list as `icloud_private_relay`, downloaded once a day, and `flags.is_relay`, true when an address is on it. Loaded, the list adds about 0.3 MiB to the Go heap; parsing it allocates about 94 MiB, freed by the next garbage collection.

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

Model: opus-5-5

TODO.md

@@ -53,6 +53,9 @@ problem the run finds.
 
 # Completed Steps
 
+- 2026-11-10: the IP lookup checks the `icloud_private_relay` list of Apple's
+  iCloud Private Relay networks, downloaded once a day. Each result has
+  `flags.is_relay` (closes https://git.eeqj.de/sneak/routewatch/issues/237).
 - 2026-11-10: the IP lookup checks the `protonvpn` list of ProtonVPN's /24
   networks and the `protonvpn_entry` list of its servers' IPv4 and IPv6
   addresses, from a public mirror of Proton's server list (closes