Single source of truth for e↔g ellipticity converters (cs_util.shape) #73

The ngmix v2.0 column grammar (#761 / branch refactor/psf-column-grammar) requires a single home for the distortion (e-type) ↔ reduced-shear (g-type) conversion: ShapePipe's HSM moments are e-type, ngmix is g-type, and consumers (sp_validation) sometimes need one given the other. Today the factors are re-derived ad hoc; they should live in one tested place that both ShapePipe and sp_validation import.

Implemented in CosmoStat/cs_util#69 — a new cs_util.shape module (parallel to cs_util.size) with component-wise e_to_g / g_to_e. ShapePipe will track cs_util @ develop so the helper is available without shipping a new cs_util release.

This is a prerequisite for finishing ngmix v2.0: the grammar's E*/G* split and the HSM e/g mislabel fix both lean on having the converter in one canonical place. Tracked here under the ngmix v2.0 epic so the dependency is explicit. Implemented and closed by #69 (this issue closes when that PR merges to develop). ShapePipe then adopts the converter — cs_util @ develop dep + import cs_util.shape — as part of the column-grammar refactor, tracked there, not here.

— Claude on behalf of Cail