Add an optional IP-to-country lookup behind a cmake option

[jagerman]

Every client draws a path screen naming the country of each hop, and each of them currently ships and maintains its own geo database to do it. This puts the lookup upstream: session::ip_country::lookup(ipv4) returns an ISO 3166-1 alpha-2 code, with available(), attribution() and database_version() alongside it.

It is off by default. With WITH_IP_GEOLOCATION off the compiled-in database is an empty one rather than absent, so every lookup misses and a client needs no #ifdef of its own; available() is a link-time fact, not a macro, so nothing about the option reaches a header. data.cpp and no_data.cpp define the same accessors and the lookup itself is identical either way -- only the table it searches differs.

DB-IP Lite is the source, chosen over MaxMind's GeoLite2 on licensing rather than accuracy: CC BY 4.0 permits redistribution and has no clause requiring a copy to stay current, which is what makes a bundled snapshot viable at all. Its IPv4 rows already tile the address space with no gaps and no mergeable neighbours, so a range needs only its first address -- the next range's start ends it. That leaves parallel arrays of 357k ipv4 starts and uint8_t country indices plus a 246-entry code table: 5 bytes a range, 1.79MB, of which a binary search touches only the 1.43MB of starts.

The codes are numbered by descending range count. The countries holding the most ranges get the shortest indices, which is worth ~0.4MB of generated source, and the rare countries -- the ones that come and go between releases -- land at the end where nothing follows them to renumber. Numbering them alphabetically instead rewrote 86% of the table across the Aug->Sep refresh, where two mid-alphabet countries disappeared; this ordering rewrote 9%.

The generated table is not committed: utils/update-ip-country-db.py downloads a release and generates it, and cmake refuses to configure with the option on until it has been run. Nothing downloads during a build. The tests run in both configurations and check the mechanism -- table invariants, range boundaries, the unknown path -- rather than pinning countries, save for one anchor commented as expected to move when the snapshot does.