Add proxy links to AP objects via FEP-fffd

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@silverpill recently pointed me to @ar-nelson's https://socialhub.activitypub.rocks/t/fep-fffd-proxy-objects/3172 , which defines some simple rules for using Links with optional rel-alternate/canonical in AP objects to formalize bridging relationships with their copies in other places. Exactly what we're doing here. Let's add them!

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FEP: https://codeberg.org/fediverse/fep/src/branch/main/fep/fffd/fep-fffd.md

Lots of interop testing discussion in the socialhub thread, specifically multi-valued and non-http url values. Seems mostly ok but probably still a bit fragile across the ecosystem.

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Done! Still worried about interop here, I don't know how much the ecosystem has advanced in supporting multi-valued url in the last couple years, but I'll watch and see.

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Working with Mastodon at least! https://browser.pub/https://bsky.brid.gy/convert/ap/at://did:plc:3ljmtyyjqcjee2kpewgsifvb/app.bsky.feed.post/3lvcb3tswns26

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https://console.cloud.google.com/errors/detail/CI7V5rLOyaWBSg;locations=global;time=P30D?project=bridgy-federated&inv=1&invt=Ab4rRw looks related, fediverse servers that aren't expecting multi-valued url are fetching the full list from us, eg https://bsky.brid.gy/r/https://bsky.app/profile/esenis.bsky.social,[object%20Object]. User-Agent is just node. 🤷