[aagbsn]

Needs decision, optional. This is one way to break ties between URLs of equal weight. We haven't chosen between it, a random tie-break, none at all, or an explicit confirmation target in the counting formula.
#1294 doesn't depend on it.

compute_priorities sorts URLs by weight alone, so URLs with the same weight come out in the same order for every probe in a country and network. Most URLs tie while the measurement counts read as zero. That's always the case for the OONI Probe app until #1294, and it happens for other probes whenever merges reset counters_asn_test_list. Background runs test only the top 20 URLs (100 when charging), so probes on a network keep testing the same URLs.

This breaks ties with a shuffle seeded by the probe's country, network and UTC day. The shuffle runs before the stable sort by weight, so the ranking is unchanged.

Weights are recomputed from the counts on every request, so a URL leaves the tie once its measurements are counted. The seed only orders the URLs that still tie:

• Probes that check in before each other's results are counted get the same tied URLs, so they repeat each other's measurements. A fully random tie-break would spread them apart.
• Later probes on the same network carry on down the tied URLs in the same order, rather than each picking at random.
• Other networks in the country use a different order.
• While the counts don't change, as for the app today or after a merge resets them, the order stays the same for the whole day and changes the next day. TIE_BREAK_SLOT_SECONDS sets that slot.

Implementation details:

• The entries come from a query without ORDER BY, so they are sorted by URL before the seeded shuffle. Otherwise the same seed could give different orders.
• Without a seed, compute_priorities keeps the entries' order, which is how show_countries_prioritization uses it.

The first reactive test list broke ties with RANDOM() (080ac01); c5f1623 dropped it when it introduced the weight formula.

Tests check that:

• the same seed gives the same order whatever order the entries arrive in (this fails without the sort);
• other networks and other days give other orders;
• the highest weight stays first;
• tie_break_seed changes per network and per day.