astronomix._fluid_equations._enforce_positivity_pallas

Pallas backend for _enforce_positivity.

Per-cell pointwise op (no halo) with input_output_aliases={0: 0} so the floored state is written back into the input buffer. Removes one full-state intermediate per stage on the FD time-integrator hot path.

Mirrors _enforce_positivity.py; the developer never touches this file by hand. See pallas_backend_implementation_guide.md §4.4 for the pallasify recipe.