fix: wire facet_rows/facet_cols roles to score plots

[eilidhmacnicol]

Goal 4 — complete a half-wired feature (pre-1.0 bug fix)

facet_rows and facet_cols group roles were accepted and validated by the YAML parser (VALID_ROLES, GroupSpec.facet_col_wrap) but nothing in the pipeline consumed them — a config requesting faceting validated cleanly and then produced an unfaceted plot, with no error. This completes the wiring rather than removing the feature.

Design

A FacetGrid is 2D and each latent variable is a distinct result that must always be shown, so LV occupies one axis and at most one demographic facet takes the other:

• Default: col = LV — unchanged behaviour.
• facet_rows: G: col = LV, row = G.
• facet_cols: G: col = G, row = LV (LV is no longer hardwired to columns — addresses the "let the user put LV on rows" case).
• Both roles: would need three axes → rejected with a clear ValueError.

facet_col_wrap controls LV-column wrapping in the default layout (inert once a row facet exists, per FacetGrid's constraints).

Changes

• io.GroupConfig: facet_rows_column(), facet_cols_column(), facet_col_wrap() accessors (mirroring hue_column()/x_axis_group()).
• pipeline._plot_score_boxstrips: resolve the grid layout from those roles and pass row_col/col_wrap through (previously hardwired col_col="LV").
• Docs: faceting section in docs/usage.md; CHANGELOG fix entry.

Tests (written first, watched fail)

• test_io: facet accessors return the role columns / None.
• test_plotting: real FacetGrid gains the expected row/col dimensions (proves the wired params aren't no-ops) — incl. LV-on-rows.
• test_pipeline::TestFacetWiring: the pipeline maps each config to the correct col_col/row_col; both-roles raises.

All 172 tests pass; ruff clean; coverage 98% (floor 95%).

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