Load site via http first, fetch via IPFS in background

I'd like to be able to run companion in "upgade my experience where possible mode"... I'd like to know when a site has an IPFS version available, and that I could pin it or switch to ti if I wanted. Redirecting all sites that have a DNSLink generally makes my browsing experience slower. In the case of the http://tableflip.io we added our site to a local ipfs node some months ago, and have since cleared out our local ipfs stores a few times due to dev work, so now I can't load the site unless I kill the redirect.

I feel like a nicer experience would involve trying to load the site over both http and ipfs, and where both respond but http wins, signalling to the user that an IPFS version is available.

Something where you can "make this site available offline and re-host it" as an opt-in rather than an eager redirect.

No files, tests, or entry points are named. Start by tracing the extension's current DNSLink redirect flow and how it handles HTTP navigation, then define the opt-in behavior described here: load HTTP first, detect an available IPFS version, and let the user fetch, pin, switch to, or re-host it for offline use without eager redirection.

Written by the indexing model from the issue text.