Refactor group analysis CLI: replace contrast_column/values with formula-based model + pairwise contrasts

[Copilot]

The old group-level CLI was limited to two-group t-tests via --contrast_column/--contrast_values. This replaces it with a statsmodels formula-based approach supporting arbitrary models, multiple pairwise factors, and stratified contrasts.

New CLI

pixi run spimquant /bids /out group \
  --model "metric ~ C(treatment) * C(genotype) * C(sex) + age" \
  --pairwise treatment \
  --within genotype sex \
  --cores all

Old	New
--contrast_column treatment	--model "metric ~ C(treatment) + age"
--contrast_values control drug	--pairwise treatment
(no strata support)	--within genotype sex

Statistical approach

• Fits a single global OLS model per region/metric (formula passed as-is, no magic expansion)
• Pairwise contrast stats (t-stat, p-val, Cohen's d) computed via marginal mean predictions + model covariance matrix
• --within strata fix specific factor values for marginal mean evaluation; model is still fit on all data

Output naming

Contrast identity is encoded in a contrast BIDS entity:

contrast-treatment+controlvsdrug_groupstats.tsv
contrast-treatment+controlvsdrug+genotype-WT+sex-M_groupstats.tsv   # with --within

Pairwise contrast labels are enumerated at Snakemake planning time by reading participants.tsv, so all outputs are statically deterministic.

Key changes

• snakebids.yml: Replaced --contrast_column/--contrast_values with --model, --pairwise, --within
• Snakefile: Reads participants.tsv at DAG-planning time to enumerate contrast labels; updated wildcard_constraints and all_group_stats/all_group_stats_coloc target rules
• groupstats.smk: Added contrast={pairwise_contrast} entity to perform_group_stats, create_stats_heatmap, map_groupstats_to_template_nii; removed concat_subj_parquet_contrast, group_counts_per_voxel_contrast, group_coloc_counts_per_voxel_contrast, concat_subj_segstats_contrast, map_groupavg_segstats_to_template_nii
• perform_group_stats.py: Rewritten — statsmodels OLS + patsy design matrix for contrast SE, replaces direct scipy.stats.ttest_ind
• Deleted: concat_subj_parquet_contrast.py, concat_subj_segstats_contrast.py

Breaking change: old --contrast_column/--contrast_values arguments are removed entirely.