Notes carrying the key. Sort key.
Up to three short example values, for judging what the key holds.
The key, NFC-lowercased. Lookup folds keys the same way, so Status and
status are one key to every tool that reads them and are reported as one
here — a caller can use either spelling.
Value shapes observed, sorted — string, number, boolean, list, map, null.
One frontmatter key as it actually occurs across the vault.