Reconsider type & field visibility modifiers

Most types are `public` (`MediaReference`, `Playlist`, `TappableSpacer`, ...), some are internal (`ExportablePlaylistDocument`, `ConfirmationGatedButton`, `Player`, `MediaPlayerView`). `NotFound` is `public` with an `internal init`. `TappableSpacer` is `public` with an `internal init`.

In a single app target none of it matters functionally. It matters if pieces are ever extracted into packages — which the `JSONDocumentStore`-as-SHELF-seam comment suggests is on the table.

Start by inventorying the named types and the JSONDocumentStore-as-SHELF-seam comment, then compare their current public and internal visibility. Done means the project has an agreed visibility policy for possible package extraction and the affected modifiers consistently follow it.

Written by the indexing model from the issue text.