feat: add Graph module with comprehensive graph data structure support (#5372)

Author: schickling

.changeset/add-graph-module.md

@@ -0,0 +1,25 @@
+---
+"effect": minor
+---
+
+Add experimental Graph module with comprehensive graph data structure support
+
+This experimental module provides:
+- Directed and undirected graph support
+- Immutable and mutable graph variants  
+- Type-safe node and edge operations
+- Graph algorithms: DFS, BFS, shortest paths, cycle detection, etc.
+
+Example usage:
+```typescript
+import { Graph } from "effect"
+
+// Create a graph with mutations
+const graph = Graph.directed<string, number>((mutable) => {
+  const nodeA = Graph.addNode(mutable, "Node A")
+  const nodeB = Graph.addNode(mutable, "Node B")
+  Graph.addEdge(mutable, nodeA, nodeB, 5)
+})
+
+console.log(`Nodes: ${Graph.nodeCount(graph)}, Edges: ${Graph.edgeCount(graph)}`)
+```