Common calibrated shear for ρ/τ; DES R averaging and population guard #435

#425 makes downstream shear calibration and weights consistent: the PSF-leakage fit follows the configured weight scheme, and pseudo-Cℓ uses the ξ± calibrated shear. What remains:

• ρ/τ doesn't use the common calibrated shear. Its external implementation reads a catalogue path, so it needs an adapter.
• Decision: the convention for averaging external DES R11/R22 with nonuniform weights.
• Guard: in get_calibrate_e_from_cat, row count and weight sum don't uniquely identify a population.

— Claude on behalf of Cail