Atlas error: WriteConflict inside a transaction

# WriteConflict in transactions

Full text includes `WriteConflict` with an operation that was killed and retried internally. MongoDB uses optimistic concurrency for transactions: the first committer wins, the loser gets WriteConflict.

## Confirm

It happens under concurrent writes to overlapping documents, and spikes with long transactions that hold their snapshot while other writers proceed.

## Fix

Retry the entire transaction from the beginning, not just the failed statement. The standard pattern:

```js
async function runTxn(fn) {
  for (let attempt = 0; attempt < 3; attempt++) {
    const session = client.startSession();
    try {
      let result;
      await session.withTransaction(fn);
      return result;
    } catch (e) {
      const labels = e.errorLabels || [];
      if (labels.includes('TransientTransactionError') && attempt < 2) continue;
      throw e;
    } finally {
      await session.endSession();
    }
  }
}
```

Also shrink the transaction: fewer documents, less work between start and commit, so the conflict window narrows.

## Verify

Run the conflicting workload concurrently. Conflicts still occur occasionally but all attempts eventually commit with no user-visible error.