Inconsistencies in the `icon` and `image` properties

This same issue affects other properties as well. For example, icon and image have similar inconsistencies:

• ActivityStreams Vocabulary: Both have range Image | Link
• JSON-LD Context: Both are defined with @type: "@id"
• Real-world usage: Often used with bare URL strings (e.g., "icon": "https://example.com/avatar.png")

The W3C primer acknowledges this:

assume that a bare string is the href of a Link object, not an id

But JSON-LD processors automatically treat @type: "@id" strings as IRI references.

The primer also notes that this issue is already recognized:

Note that, for linked-data-aware consumers, the @type of the url property is not strictly aligned with the text of the specification. Namely, it requires an @id value, which may not match with the xsd:anyUri option mentioned in the text.

This creates implementation challenges for ActivityPub libraries that rely on proper JSON-LD processors (like Fedify), which automatically attempt to dereference these URLs as ActivityStreams objects rather than treating them as Link hrefs.

See the discussion at https://github.com/fedify-dev/fedify/issues/420 for a concrete example where this causes parsing errors.

The affected properties likely include all those with:

• Range of Object | Link (or similar combinations)
• Current JSON-LD context definition of @type: "@id"
• Common real-world usage as bare URL strings

Would it make sense to address these properties together with href?

Originally posted by @dahlia in #595