nwt-substrate: a substrate-algebraic computation library for Null Worldtube Theory

Algebra-to-picture color mapping for Feynman diagrams The Diagram class can now express the structural correspondence between each chunk of a Feynman amplitude iℳ and its associated diagram element (incoming/outgoing fermion lines, internal propagators, vertex factors), and render both with matched coloring. What's new Term dataclass (nwt_substrate.qed.diagram.Term): a single chunk of the algebraic amplitude paired with the diagram element it represents (element_id) and a color. Diagram.expression_terms / element_colors: optional decomposition of the amplitude into colored chunks + a registry mapping element ids to colors. Diagram.render_color_mapped(...) / Diagram.save_color_mapped(...): render the diagram…