detect_volatile_domains() was resolving every domain via dig on each run,
which took 30+ minutes for large lists (e.g. 2097 whatsapp domains).

Cache domains confirmed as non-volatile (stable TTL, same IPs from
different resolvers) and skip them on subsequent runs. Cache expires
after 7 days and is ignored with --force.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>