BatchOpalSurface

What the `batch.*` OPAL tasks are allowed to reach (batch-orchestration §BOO-1).

In the namespace batchtasks. Declared in src/libs/libbatchopal/batchopal/opal/BatchOpalSurface.hpp.

Detailed description

Four references, and every one of them may be null. A node that runs jobs and joins no fleet has a queue and an execution manager and nothing else, and its `batch.executors.*` tasks must say so rather than crash — the headless coordinator this surface exists for is the case where nobody is watching when it does.

The queue is shared because the tasks outlive any one caller. The rest are borrowed: the execution manager is a `QObject` owned by whatever built the dispatcher, and QPointer makes its destruction visible instead of undefined.