validate_stratpal: Validate a stratigraphic palette
Description
Ensures that a palette covers all required categories. By default,
missing categories trigger an error. If allow_na = TRUE, missing
categories are filled with a default color instead.
# A complete palette passes validationvalidate_stratpal(stratpal_rpg)
# Allow missing categories to be filled with grayvalidate_stratpal(stratpal_grays, allow_na = TRUE)