Add section in "Object identifiers" about how to specify secondary identifiers for an object

[evanp]

We had a meeting of the WG to discuss this topic:

https://github.com/w3c/socialwg/blob/9bb70c4b5ee4a147facc86ea2e57a69c8322868b/meetings/2026/2026-07-03-CG-WG-joint.md?plain=1#L79

We want to open up space to explore ideas like #573 without breaking interoperability with existing Federated Servers. One option is to continue to recommend HTTPS URIs for the id property, but to allow other URI protocols in another property.

We discussed at least these properties:

• alsoKnownAs: this is used by Mastodon and other for data portability; see https://swicg.github.io/activitypub-dataportability/
• url: this is usually used for finding non-AS2 documents related to an object, like image files or HTML profile pages
• owl:sameAs: This might be the best candidate, since semantically it fits exactly what we're looking for

Regardless, if we want to encourage this kind of exploration, having a principled way to do it in this section is the first step.

[evanp]

@snarfed notes that the at: URI of a Bluesky post is included in the bridged content into the Fediverse using the proxy objects structure in FEP FFFD. This uses url with a rel of alternate.

One thing I want to make sure of is if we need to have different ways of saying that "this is an alternate representation of the 'same thing'", or "this is an alternate way of getting a binary-identical copy of this AS2 document". Is this important and meaningful?

[evanp]

We also discussed bridges and archives using an annotation mechanism to provide additional identifiers. "I bridged this AP Note to this AT URI: ...". "I archived this AP Note to this IPFS URL: ...". Those annotations can be shared back to the originating server or to downstream subscribing servers.