A Shear Bound from the Isotropy of Polarisation Rotation: Direction-Dependent Angles in an Anisotropic Bounce

This paper examines what limit the observed isotropy of the rotation angle places on direction-dependent effects at a bounce, under the stated assumption that any such effect couples linearly to a dimensionless shear parameter with a coefficient of order unity. The calculation is performed from stated assumptions and published polarisation measurements, using the deposited script so that every quoted number can be reproduced. The headline outputs are: dispersion limit deg = 0.22; shear bound = 3.84e-03; dispersion at s0p01 deg = 0.573. The treatment separates what the published measurements supply from what a theoretical framework must additionally provide; where a required quantity is not supplied, that absence is reported plainly as a negative result rather than replaced by an assumed value. The study is computed from stated assumptions and published inputs; it is not a new measurement. SIMULATED, NOT MEASURED in the sense that all derived quantities are calculated, not observed.