Attribute resolution and intermediate representation

The parsed AST is a syntactic representation of the DOT file. The renderer needs a semantic intermediate representation (IR) that resolves attribute inheritance, deduplicates nodes, and builds an adjacency structure suitable for layout algorithms.

The IR consists of IrGraph (nodes, edges, clusters, graph-level attributes), IrNode (id, label, resolved attributes, position/size slots for layout to fill), IrEdge (from/to indices, label, resolved attributes, routed points for layout to fill), and IrCluster (node indices, sub-clusters, bounding box for layout to fill). Attributes are stored as a string map with typed accessor methods for known attributes like shape, color, and fontsize; unknown attributes are silently ignored.

The AST-to-IR lowering pass walks statements in order, maintaining a scope stack with default attribute maps for graph, node, and edge targets. AttributeStatement updates the current scope's defaults. Nodes are deduplicated by ID, with later attribute declarations merged in. Edge chains like A -> B -> C are expanded into individual edges. Subgraphs whose IDs start with cluster_ produce IrCluster entries. Subgraph edge targets are expanded to edges from/to all nodes in the subgraph.

Start by locating the parsed AST, renderer entry point, and AST-to-IR lowering pass described in the issue. Review how statements, subgraphs, attributes, and edge chains are represented before designing the IR. Done means the lowering pass resolves inherited attributes, deduplicates nodes, expands edges, and produces the specified graph, node, edge, and cluster structures.