Expose fine-mapping credible-set knobs + L/L_greedy defaults in the GWAS RSS workflow

[Yining97]

Summary

Surfaces pecotmr::fineMappingPipeline's credible-set controls through the GWAS RSS workflow, which
previously only forwarded --methods, --coverage, and --method-args.

• fine_mapping.R: new --min-abs-corr, --pip-cutoff, --secondary-coverage, and
  --median-abs-corr flags, passed to fineMappingPipeline (the last only when set, so the script
  still runs against a pecotmr that predates that argument). New --L / --L-greedy default the
  SuSiE fit to 20 / 5, seeded into the per-method kwargs with --method-args overriding per key.
• gwas_rss_fine_mapping.ipynb: exposes all of the above as [global] parameters and forwards them
  in the gwas_fine_mapping step.

Validation

Against the matching pecotmr (below) on real chr21 ADSP LD + AD_Bellenguez: full sos run completes;
--min-abs-corr and --median-abs-corr both tune the recovered credible sets (1→2); --method-args
L-override changes the fit; the top_loci carries af; and a sos run matches a direct
fine_mapping.R call (PIP / af / CS identical).

Depends on

--median-abs-corr and the af output column require the pecotmr change in
StatFunGen/pecotmr#530 (and a susieR whose susie_get_cs has median_abs_corr). Merge that first.