feat!: return indexed subgraph result with adjacency maps

[pdlug]

• BREAKING: store.subgraph() now returns { root, nodes: Map, adjacency, reverseAdjacency } instead of { nodes: Node[], edges: Edge[] }
• Eliminates the adjacency-index-rebuilding boilerplate every consumer writes before traversing a subgraph result
• Forward and reverse adjacency maps provide O(1) edge lookup by nodeId → edgeKind
• root provides direct access to the traversal starting point

Migration

// Before
const nodeById = new Map(sg.nodes.map(n => [String(n.id), n]));
const items = sg.edges.filter(e => e.kind === "has_item" && e.fromId === orderId);

// After
const item = sg.nodes.get(itemId);
const itemEdges = sg.adjacency.get(orderId)?.get("has_item") ?? [];
const parentEdges = sg.reverseAdjacency.get(itemId)?.get("has_item") ?? [];

Closes #82