ShapePipe — longer-term improvements (not blocking the ngmix v2.0 merge)

[cailmdaley]

Umbrella for ShapePipe improvements that are not prerequisites for merging the ngmix v2.0 PR (#741 / epic #762). #762 holds the must-fix-before-merge work; this epic holds everything worth doing that can land after, on its own cadence.

Current members:

• ngmix centroid: switched to WCS in #741 — confirm correct, and any reason to keep HSM? #767 — ngmix centroid: confirm the WCS switch in Ngmix v2.0 (CI mirror of #740) #741, and whether there's any reason to keep HSM.
• ngmix: the reduced-χ² anomaly on stars (error model) #769 — ngmix: the reduced-χ² anomaly on stars (error model).
• HSM: measure PSF/star shapes directly in sky coords (use_sky_coords), drop the WCS re-transform #775 — HSM PSF/star shapes directly in sky coords (use_sky_coords on FindAdaptiveMom), dropping the WCS re-transform.
• UberSeg neighbour masking (Erin Sheldon) — config-selectable, validated against noise-fill #776 — UberSeg neighbour masking (Erin Sheldon), config-selectable, validated against noise-fill.
• ngmix PSF fitter: drop the galaxy prior, adopt a metadetect-style dedicated PSF fit #778 — ngmix PSF fitter: drop the galaxy prior, adopt a metadetect-style dedicated PSF fit (Axel's suggestion on [BUG] ngmix module passes the galaxy shape prior to the PSF fit, which is prior-dominated #749).

(GitHub also tracks these as sub-issues; new enhancements get added as they surface.)

— Claude on behalf of Cail