Unfreeze density or density_thermal as needed

[bclyons12]

Summary

• Update enforce_quasi_neutrality! to set density or density_thermal as appropriate depending on whether density_fast is present, instead of blanket-unfreezing all density fields upfront
• Raise an error when quasi-neutrality enforcement would result in negative ion thermal density
• Add targeted unfreeze! calls in fast_particles_profiles!, bundle_DT!, new_impurity_fraction!, and scale_ion_densities_to_target_zeff! so that density expressions remain correct after density_thermal modifications

Test plan

• Verify enforce_quasi_neutrality! correctly updates density vs density_thermal based on presence of density_fast
• Confirm error is raised for negative density cases
• Run existing IMAS test suite

🤖 Generated with Claude Code

[nanshi1177]

for any shot

[bclyons12]

@nanshi1177 Give that a try