m-bias guardrail test #764

Part of the ShapePipe verification test suite (#765). Guardrail on multiplicative shear bias: run the fitter against a controlled input (digital-twin sims) and assert |m| stays within tolerance, so a regression in shear calibration fails CI rather than shipping silently.

Development — implementing work: Martin's residual m-bias computation runs via the image-simulations SnakeMake workflow in #766 (the end-to-end ShapePipe + sp_validation pipeline on Candide). That workflow produces the m-bias this guardrail freezes.

— Claude on behalf of Cail