feat(8.0): related({ node }) — one-call both-direction incident edges

related({ from }) / related({ to }) each return one direction; related({ node })
unions both (every edge where the node is source OR target), deduped, in a single
O(degree) call — the indexed equivalent of merging two related() calls by hand.
The 'all edges on a noun' query the graph-viz path needs.

- RelatedParams.node: mutually exclusive with from/to (throws if combined);
  composes with type/subtype/visibility filters; natural-key ids resolved.
- Implemented as a parallel out-edge (sourceId) + in-edge (targetId) fetch through
  the O(degree) graph-index fast paths, merged + deduped by id (a self-loop appears
  once), sorted for a stable page. Full incidence is the intended O(degree) cost;
  deep offset paging of a very-high-degree node is best-effort (use the native
  edgesForNode / a graph cursor for exhaustive paging).
- db.related() parity: node resolved + honored in relationMatchesParams (matches an
  edge incident in either direction) for the historical/overlay paths.

Test: related-node-bidirectional.test.ts — both directions, equals from∪to, self-loop
dedupe, type-filter composition, default-hides-internal, node+from/to throws.

src/db/db.ts

@@ -460,7 +460,8 @@ export class Db<T = any> {
     const params: RelatedParams = {
       ...rawParams,
       ...(rawParams.from !== undefined && { from: resolveEntityId(rawParams.from) }),
-      ...(rawParams.to !== undefined && { to: resolveEntityId(rawParams.to) })
+      ...(rawParams.to !== undefined && { to: resolveEntityId(rawParams.to) }),
+      ...(rawParams.node !== undefined && { node: resolveEntityId(rawParams.node) })
     }
     const historical = this.isHistorical()
 
@@ -1024,6 +1025,10 @@ function sortResultsBy<T>(results: Result<T>[], orderBy: string, order: 'asc' |
  * `to` → targetId, type/subtype set membership, service equality).
  */
 function relationMatchesParams<T>(relation: Relation<T>, params: RelatedParams): boolean {
+  // `node` matches an edge incident in EITHER direction (the both-direction shorthand).
+  if (params.node !== undefined && relation.from !== params.node && relation.to !== params.node) {
+    return false
+  }
   if (params.from !== undefined && relation.from !== params.from) return false
   if (params.to !== undefined && relation.to !== params.to) return false
   if (params.type !== undefined) {