Cap HMCode2020 feedback stellar fraction at baryon fraction

[Uendert]

This PR adds the HMCode2020 feedback stellar-fraction cap used in the original Fortran HMcode implementation.

For HMCode2020 feedback, the stellar fraction should be capped at the cosmic baryon fraction:

fstar <= Omega_b / Omega_m

Without this cap, the Julia implementation can exceed the baryon fraction at high redshift, producing large feedback deviations.

This PR adds the same cap and a regression test checking that:

• low-redshift values are unchanged when fstar < Omega_b / Omega_m
• high-redshift values are capped when fstar > Omega_b / Omega_m

Validation:

• the new targeted feedback regression test passes locally
• the broader existing test suite starts correctly after declaring the standard Julia Test target