fix: render heatmap metadata colour bars

[eilidhmacnicol]

Goal 5 — complete a half-wired feature (pre-1.0 bug fix)

plot_lv_heatmap and plot_bootstrap_heatmap pass row_colors/col_colors (the --x-meta/--y-meta category colours) into plot_heatmap, but plot_heatmap builds the figure with sns.heatmap, which has no row/column colour-bar support — so the colours were silently dropped and the metadata colour-coding never appeared on --all-plots heatmaps. This completes the wiring rather than removing it.

Approach

When colour bars are requested, render them via sns.clustermap(row_cluster=False, col_cluster=False, row_colors=..., col_colors=...) — a non-clustered heatmap that does support colour strips, with the empty dendrogram axes hidden. The diverging vlag scale, symmetric [-v, v] range, and annotation-suppression behaviour are preserved.

Heatmaps without metadata are unchanged — they keep the original sns.heatmap path (used by the cross-correlation heatmap and metadata-free bootstrap heatmaps). Only the previously-broken colour-bar path changes.

Changes

• plotting.plot_heatmap: branch to a new _heatmap_with_colour_bars helper when row_colors/col_colors are present.
• Docs: note the colour bars in docs/usage.md; CHANGELOG fix entry.

Tests (written first, watched fail)

• test_plotting: LV heatmap renders both row and column colour bars; bootstrap heatmap renders a row colour bar only (columns are LVs, no metadata); the no-colour path does not invoke clustermap (behaviour-unchanged guard). Assertions check the colour-bar axes actually drew artists, not just that a file was written.

All tests pass; ruff clean; coverage 97.8% (floor 95%). Verified the output visually — the row/column category bars render with the correct colours.

🤖 Generated with Claude Code