test: add fp32/fp64 dtype consistency regression tests

25 tests pinning the precision claims in src/mdpp/_dtype.py so any
future refactor that silently changes the dtype policy is caught
on either side: fp32 paths developing visible drift versus fp64,
or fp64 paths someone tries to demote and lose load-bearing
precision.  Covers RMSF, DCCM, RMSD, distances, FES, PCA, TICA,
contacts, h-bonds, RMSD matrix, K-Means, average/delta-RMSF SEM
propagation, plus the default-dtype contract and invalid-dtype
rejection.

Author: FridrichMethod