Look up a physical constant by its key (e.g. "speed-of-light"), symbol (e.g. "c") or exact name. Returns the CODATA 2018 recommended value, unit, relative standard uncertainty (0 for values that are exact by SI definition) and category.

List every physical constant in the dataset, or only those in a given category (universal, adopted, electromagnetic, atomic, physico-chemical). Returns the constants, the total count and the set of available categories.

Search the constants table by key, symbol or name substring. Exact matches rank first, then prefix matches, then substring matches. Returns up to `limit` constants (default 20) plus the total match count.