Function makeEnqueueTransaction

Builds a "single-instance transaction queue":

• All entry points (dispatch/source-refresh/...) execute serially through the same FIFO queue.
• Callers still experience the entry as a single Effect (preserving the existing API shape).
• Tasks must "never fail", otherwise the queue consumer fiber would deadlock (so we return results via Deferred/Exit).

NOTE: transaction execution happens inside a background queue fiber. To support Provider-local overrides (Tag/Layer) and diagnostics tiers (FiberRef) at the call site, we capture minimal context at enqueue-time and re-provide it to the task.