`category`:
  * "shift_range" -- typical shift window for a functional group, for
    `nucleus` (1H or 13C). A nucleus with no table returns an empty list
    rather than silently falling through to the proton table, so an
    unsupported nucleus is visible instead of answered ~120 ppm wrong.
  * "coupling"   -- J-coupling ranges by type (vicinal, geminal, ...).
  * "solvent"    -- residual solvent peaks; one record per solvent with its
    1H / 13C / 19F / 31P shifts. NOTE this category returns a single object
    in `results`, not a list.
  * "all" (default) -- searches every table and returns grouped matches.

An empty `query` is a legitimate "show me the table" request. Bounded at
`max_results` (50) per category; `n_results` is the untruncated count.