Senders on a subdomain that hosts no favicon (e.g. mail.example.com) left
feeds blank because both the direct /favicon.ico and the DuckDuckGo lookup
were tried only against the full subdomain. Resolution now walks up to the
apex via Domain.parents() and caches the result under the original sender
domain.
Co-Authored-By: Claude Opus 4.7 <[email protected]>