Fixes to Python module

[astrofrog]

This fixes a few bugs with the Python package:

• PowerLawEnvelope power = -3 — the mass↔ρ₀ conversion computed (rmax**alpha - rmin**alpha) / alpha with alpha = 3 + power, dividing by zero at power = -3. Routed through integrate_powerlaw, which handles the limit.
• FreezableClass per-instance attributes — the attribute registry was a class-level list shared by every instance, so registered attributes leaked between instances (and the list grew unbounded), defeating the frozen-attribute check. Now per-instance; also declares AmbientMedium.star before freezing (required once the registry is per-instance).
• zero_density (AMR) — zmin defaulted to +inf, so the reset mask covered every cell and zeroed the whole grid; it also referenced a nonexistent data attribute. Now defaults sensibly, operates on the grid's quantities, and returns the grid passed in.
• Voronoi evaluate_function_average — divided the last cell's sample sum by a decremented count, biasing (or zeroing) that cell's average. Count corrected.
• _discretize_sph C extension leaks — _get_positions_widths never released its input array and returned its six output arrays with "O" (which adds references) instead of "N" (which steals them), leaking all seven arrays per call. Reference handling corrected, plus error-path cleanup.
• Cylindrical auto-grid z walls — built n_z walls instead of n_z + 1, producing one fewer cell than requested and failing outright for small n_z. Now produces exactly n_z cells and reaches zmax.