feat(groups): binary control-vs-user grouping (drop trauma 3-way split)

DMR no longer filters user_unknown; group comes from the samplesheet 'group' column, now binary (control/user). The former user_trauma/user_notrauma/user_unknown collapse to 'user' (trauma is no longer a grouping factor); DMR runs the single control-vs-user pair, entropy over both groups. Localize/motif/MERGE unchanged (they read group straight from the samplesheet). Docs updated. NOTE: the samplesheet itself (PII, gitignored) is remapped on the cluster (group = cohort).