`localQueue` breaks the documented serial execution guarantee for named queues

[anavalo]

The docs state that jobs added to the same named queue run "in series", "one at a time". With worker.localQueue enabled they run concurrently — per-queue concurrency becomes min(localQueue.size, concurrentJobs) instead of 1. A single worker pool is enough to trigger it.

Nothing errors, retries or logs; every job reports success. An application relying on that guarantee to serialise read-modify-write work will silently lose writes.

Reproduced on 0.17.3 and on main (cea9d60).

Reproduction

docker run -d --name gw-repro-pg -e POSTGRES_PASSWORD=postgres \
  -e POSTGRES_DB=gwrepro -p 54329:5432 postgres:16-alpine
npm init -y && npm i graphile-worker@0.17.3
node repro.js

const { run, makeWorkerUtils } = require("graphile-worker");

const connectionString =
  process.env.DATABASE_URL ||
  "postgres://postgres:postgres@localhost:54329/gwrepro";
const sleep = (ms) => new Promise((r) => setTimeout(r, ms));

(async () => {
  const utils = await makeWorkerUtils({ connectionString });
  await utils.migrate();

  // A backlog of 12 jobs, all on ONE named queue.
  for (let i = 1; i <= 12; i++) {
    await utils.addJob("t", { i }, { queueName: "my-queue" });
  }

  let active = 0;
  let maxActive = 0;
  const runner = await run({
    preset: {
      worker: {
        connectionString,
        concurrentJobs: 4,
        localQueue: { size: 5 }, // <-- remove this line and maxActive is 1
      },
    },
    taskList: {
      t: async () => {
        active++;
        if (active > maxActive) maxActive = active;
        await sleep(200);
        active--;
      },
    },
  });

  while (
    (
      await utils.withPgClient((c) =>
        c.query("select 1 from graphile_worker._private_jobs limit 1"),
      )
    ).rowCount
  ) {
    await sleep(50);
  }
  await runner.stop();
  await utils.release();

  console.log(`jobs from one named queue running at once: ${maxActive}`);
  console.log(`expected per the docs: 1`);
})();

jobs from one named queue running at once: 4
expected per the docs: 1

Remove the one localQueue line and the same script prints 1.

size: 5 is concurrentJobs + 1, the value recommended in docs/performance. size: 2 breaks it too; concurrentJobs: 1 with size: 16 does not.

Environment

graphile-worker	0.17.3 (npm latest), and main @ cea9d60 built from source
Node	v24.5.0
Postgres	16.14