Reads the acquisition header only -- nothing is processed. Returns the
format, 1D/2D, the nucleus, the point count, the pulse sequence the
spectrometer recorded (`experiment_declared`), what that sequence actually
is (`experiment_detected`), and a ranked list of the tools worth running.

The declared-vs-detected pair is the point: a spectrum handed to the wrong
slot is the defect covnmr_2d_analyze, covnmr_sudoku and covnmr_multi_analyze
all guard against, and this answers that question before any credits are
spent processing it.

`nucleus` here comes straight from the file header and `nucleus_source`
says so -- it is not the resolved value the analysis tools compute.