[Bug] Make S15 background-result delivery atomic #524

Problem

Concurrent consumers of S15 background-task results can race while collecting completed tasks. The same result may be delivered more than once, or result delivery can be inconsistent when background tasks finish while another consumer is collecting.

Background task ID allocation is also separate from task registration, so concurrent starts can race on the shared counter and registry.

Expected behavior

Each completed background task is delivered exactly once. Task ID allocation and task registration are atomic.

Scope

Fix the S15 implementation and add focused regression coverage. Windows-only compatibility is out of scope.