dCDH wrap-up: WLS df -> OLS df wording fix (codex R2 P3)

Single CI codex R2 P3 finding: the non-survey heterogeneity parity
path uses OLS (only the survey path uses WLS), but two recently-
added comments described its df source as "WLS df". Updated:
- REGISTRY heterogeneity Note: "df from the WLS regression" -> "df
  from the OLS regression".
- Parity test inline comment: "WLS df" -> "OLS df".

Wording drift only; no behavior or test contract change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber