Use pre-reionization x_e for cs2 to remove high‑k P(k) dependence on WantCls (fixes #171)

[cmbant]

Compute baryon cs2 using the pre‑reionization ionization fraction (xe_a) rather than the post‑reionization x_e. This removes the high‑k dependence of linear P(k) on WantCls when reionization is enabled.

Expected fractional change in linear P(k) for τ=0.05 (changed/new divided by baseline), k in h/Mpc:

• z = 0:
  • k ∈ [1, 10]: ≤ 0.002%
  • k ∈ [10, 100]: ≤ 0.21%
  • k ∈ [100, 1000]: up to ~16.7% (decrease)
• z = 3:
  • k ∈ [1, 10]: ≤ 0.001%
  • k ∈ [10, 100]: ≤ 0.12%
  • k ∈ [100, 1000]: up to ~9.3% (decrease)

Plot: docs/cs2_compare_tau005.png

No public API changes.

Fixes #171.

[cmbant]