Standard way of indicating post type

The Microsub specification currently does not define a standard way of including an item's [post type](https://indieweb.org/post-type-discovery) in a timeline response.

From what I can tell, there are currently some server implementations that include post type in their response, like [Aperture](https://github.com/aaronpk/Aperture) in the `post-type` property, but there does not seem to be consensus on how to do this (mainly due to a lack of implementations).

Wouldn't it make sense to make this part of the spec? I would consider this a part of data normalization/transformation which is literally the purpose of a server.

I suggest we use a top-level field like `_kind` (to match `_id`, `_is_read` and `_source`) as a standard way to indicate post type, `post-type` seems inappropriate since post type is part of an items metadata and not a MF2/JF2 property.

Start by reviewing the Microsub specification's timeline response format and the linked post-type discovery guidance. Compare the existing conventions mentioned in the issue, including Aperture's post-type property, and determine whether a standard field can be agreed on. Done means the specification documents the chosen representation and its relationship to item metadata.

Written by the indexing model from the issue text.