Add an example of an in-process model of distributed execution

### Is your feature request related to a problem or challenge?

DataFusion's physical plans are written and tested assuming everything runs in one process: one `Arc` instance, state shared via `Arc`, and all partitions of an operator polled together by cooperative threads on a shared runtime.

Distributed engines built on DataFusion (Ballista, datafusion-distributed, comet, and proprietary engine like at Coralogix) violate those assumptions.

### Describe the solution you'd like

Add a small educational example that runs queries in a distributed mode:

1. A example** of how distributed execution works on top of DataFusion: breaking a query into stages, serializing plans, and executing tasks in isolation.
2. A **regression guard**: once a subset of its tests runs in CI, it catches the class of change that breaks isolated execution before it ships to downstream projects.

The crate would be excluded from the workspace so it has zero impact on normal builds and published crates; it builds in CI.

### Additional context

- Draft PR with a working prototype: #23282
- Follow-up: run a subset of sqllogictests through the distributed harness for much broader differential coverage (separate issue).

Review draft PR #23282 first to understand the working prototype and its distributed harness. Verify that the excluded crate builds in CI and that the example covers staged queries, plan serialization, isolated task execution, and regression tests; done when those behaviors are exercised without affecting normal workspace builds.

Written by the indexing model from the issue text.