Create styles/nature.py with accessible 8-colour palette (Paul Tol
bright scheme), Altair theme registration, matplotlib rcParams, and
Cytoscape style helpers — all conforming to Nature Research figure
specifications (bib citation 9ASETLJ4).
Retrofit acodb_explorer.py: replace all hardcoded set2/hex colors
with PALETTE references, enable the Nature Altair theme on import.
Update src/aco/dag.py to import schema colors from the shared
styles/nature.py module instead of maintaining a separate Tableau 10
palette.
fix#45, fix#46, refs #47