make_cat: remove the dead galsim shape-serialization path #783

Carved out of #761 per Axel's 2026-07-02 review ("don't change too many things at once").

make_cat still carries a galsim shape-serialization path — _save_galsim_shapes(), GALSIM as an allowed SHAPE_MEASUREMENT_TYPE, and a test_galsim_grammar_properties.py — but no runner produces galsim shapes (there is no galsim_shapes_runner), and the dual-shape (len==5) input branch in make_cat_runner never fires. ngmix is the pipeline's only shape estimator. The path is dead code.

This removes it:

• delete _save_galsim_shapes() from make_cat.py and drop GALSIM from SHAPE_MEASUREMENT_TYPE (make_cat_runner validates against ngmix alone, raising otherwise — the knob is kept as the extension point for a future estimator family)
• drop test_galsim_grammar_properties.py and the galsim case from the frozen column-grammar examples
• lose the galsim references in the docstring and the cat_matched.param comment

The removal is good work and stands on its own; it was pulled out of #761 only so that PR stays scoped to the reconvolved-PSF column grammar.

Not blocking the ngmix v2.0 merge — the galsim path is dormant either way, so this is cleanup, not a v2.0 correctness fix. Filed under the longer-term-improvements epic (#773).

Implementing PR is stacked on #761 (refactor/psf-column-grammar) because the removal deletes a test that #761 introduces; it retargets to ngmix_v2.0 once #761 merges.

— Claude on behalf of Cail