New: poll what changed instead of re-downloading the index

This index now publishes a since-cursor feed at /changes.json. Read `cursor` from the response, send it back as ?since=, and you get only the operator IP ranges, upstream failures and crawler records that moved since then. It advances only on a real change, so an unchanged answer is proof rather than luck — about 2.5 KB, or a 304 with no body if you send back the ETag or Last-Modified. The upstreams are re-read every six hours, so that is the interval that always has news. No key, no rate limit, no webhook: the feed is the whole mechanism. curl -s 'https://www.pathwren.workers.dev/changes.json?since=0'