Add ThermoResult object for streamlined thermo-abundance workflow

[smsharma]

Summary

Closes #14

BackgroundModel now returns a ThermoResult NamedTuple that AbundanceModel can accept directly, with automatic temperature range detection.

Changes

• Add ThermoResult NamedTuple to linx/thermo.py containing all thermo outputs plus T_start/T_end
• BackgroundModel returns ThermoResult instead of plain tuple
• AbundanceModel accepts either:
  • ThermoResult object (auto-extracts all values including T_start/T_end)
  • Individual arrays (backward compatible)

Usage

# New streamlined API
thermo_result = BackgroundModel()(0.)
abundances = AbundanceModel(nuclear_net)(thermo_result)
# T_start/T_end auto-detected from thermo_result

# Backward compatible usage
t, a, rho_g, rho_nu, rho_extra, P_extra, Neff = thermo_model(0.)[:7]
abundances = abundance_model(rho_g, rho_nu, rho_extra, P_extra, t_vec=t, a_vec=a)

Test plan

• Existing tests pass
• Tuple unpacking backward compatibility verified

🤖 Generated with Claude Code

[cgiovanetti]

Thanks for putting this together--I'm going to hold off on this one because it requires us to also update the example notebooks, and I want to play with it a bit in some different contexts before pulling the trigger on this switch. I'm a little backed up trying to get the stuff on cara/me-hardcode integrated in and waiting for a response on jax-ml/jax#33666, once I clear that away I'll come back to this