"just --list" should list fallback recipes

Now that fallback is stable, it seems that `just --list` should be aware and display fallback recipes, perhaps under a "fallback" section.

Start by tracing the `just --list` command and how fallback recipes are represented. Check how the list output groups recipes, then determine where fallback recipes should appear under a separate "fallback" section. Done means `just --list` displays fallback recipes without disrupting the existing recipe listing.

Written by the indexing model from the issue text.