fix(gen): move perfect graph generators before default handling

- Move ptolemaic, modular, quasi-line checks earlier in generateBaseStructure
- Prevents these exact structures from being overridden by default connectivity handling
- Add perfect graph variants to hasExactStructure to prevent density edge additions
- Fixes root cause: ptolemaic generator creating paths, then generateDisconnectedEdges adding cycles
- Modular and quasi-line generators also benefit from this fix

This ensures graph class generators produce correct, unmolested structures.

Author: Mearman

src/generation/generators/density-helpers.ts

@@ -19,6 +19,11 @@ export const hasExactStructure = (spec: GraphSpec): boolean => {
       spec.binaryTree?.kind === "complete_binary") return true;
 	if (spec.tournament?.kind === "tournament") return true;
 
+	// Perfect graph class generators (produce exact structures)
+	if (spec.modular?.kind === "modular") return true;
+	if (spec.ptolemaic?.kind === "ptolemaic") return true;
+	if (spec.quasiLine?.kind === "quasi_line") return true;
+
 	// Regularity constraints
 	if (spec.cubic?.kind === "cubic") return true;
 	if (spec.specificRegular?.kind === "k_regular") return true;

src/generation/generators/edge-generator.ts

@@ -500,6 +500,24 @@ const generateBaseStructure = (nodes: TestNode[], spec: GraphSpec, _config: Grap
 		return handleTopologicalMinorFree(nodes, edges, spec, rng);
 	}
 
+	// ============================================================================
+	// PERFECT GRAPH VARIANTS (check before default connectivity/cycles handling)
+	// These generate exact structures that should not be modified
+	// ============================================================================
+
+	if (spec.modular?.kind === "modular") {
+		generateModularGraphEdges(nodes, edges, spec, rng);
+		return edges;
+	}
+	if (spec.ptolemaic?.kind === "ptolemaic") {
+		generatePtolemaicEdges(nodes, edges, spec, rng);
+		return edges;
+	}
+	if (spec.quasiLine?.kind === "quasi_line") {
+		generateQuasiLineEdges(nodes, edges, spec, rng);
+		return edges;
+	}
+
 	// Non-bipartite graphs
 	if (spec.connectivity.kind === "connected" && spec.cycles.kind === "acyclic") {
 		// Generate tree structure
@@ -573,20 +591,6 @@ const generateBaseStructure = (nodes: TestNode[], spec: GraphSpec, _config: Grap
 		return edges;
 	}
 
-	// Handle perfect graph variants
-	if (spec.modular?.kind === "modular") {
-		generateModularGraphEdges(nodes, edges, spec, rng);
-		return edges;
-	}
-	if (spec.ptolemaic?.kind === "ptolemaic") {
-		generatePtolemaicEdges(nodes, edges, spec, rng);
-		return edges;
-	}
-	if (spec.quasiLine?.kind === "quasi_line") {
-		generateQuasiLineEdges(nodes, edges, spec, rng);
-		return edges;
-	}
-
 	return edges;
 };