perf(javascript): reuse graph buckets and query indexes

Append overlapping observations to node buckets instead of recopying groups, and share one node index across semantic seed admission and traversal. Preserve merged provenance and absent-seed behavior.

src/domain/javascript/javascriptSemanticGraphTraversal.test.ts

@@ -316,6 +316,20 @@ it("returns every relevant unknown inline", () => {
   expect(result.unknowns).toHaveLength(1);
 });
 
+it("does not admit a semantic-node seed absent from the graph", () => {
+  const result = queryJavaScriptSemanticGraph(fixtureGraph(), {
+    seed: { kind: "semantic-node", node_id: `jsrg_node_${"f".repeat(64)}` },
+    direction: "forward-influence",
+  });
+
+  expect(result).toMatchObject({
+    status: "no-match",
+    seed_node_ids: [],
+    nodes: [],
+    summary: { total_seed_matches: 0, traversed_nodes: 0 },
+  });
+});
+
 it("returns the complete deterministic forward influence result inline", () => {
   const graph = fixtureGraph();
   const input = {