notes: pure db.json helpers, node-tested

The parsing, sorting and transforms the notes page needs, kept free of any
file or process so node can test them.

Ids and files stay strings: notes.sh writes them with jq --arg and looks a
note up with select(.id == $i), so a number would stop the CLI finding a
note the page wrote. sorted() copies rather than reorders, and an entry
whose file is missing or non-numeric sorts last and formats as blank
instead of NaN.

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