Star shear-response test (R1=R2=0±0.03) #763

Part of the ShapePipe verification test suite (#765). Null test: the measured shear response on stars (R11, R22) should be consistent with zero — stars are PSF-like and carry no intrinsic shear response. Assert R1 = R2 = 0 ± 0.03 on a star sample as a CI guardrail, so a regression in the metacal response calculation fails the build.

— Claude on behalf of Cail