fix!: Make the first snippet or origin primary

[Muscraft]

Trying to determine what was "primary" by looking for AnnotationKind::Primary or Origin::primary was a bit brittle and had rendering bugs if an Origin or Snippet came before the "primary" one. To avoid those problems and make things more consistent, I made it so the first Origin or Snippet we encounter is the "primary". This change also has the benefit of allowing us to get rid of Origin::primary in a future PR, which will make Origin rendering better into the current Group model.

[epage]

Note that as this has end-user impact, this is not a chore