Which issue does this PR close?

Closes #1727.

Rationale for this change

Neither datafusion-ffi-example nor datafusion-ffi-query-planner-example had a standalone script that users could run; they only delivered pytest assertions. Meanwhile, the documentation explicitly promises "a complete example" and a "user example." This PR provides standalone runnable scripts for both crates, ensuring they behave like real examples. It follows the examples/tpch precedent: the script is the primary artifact, and a subprocess pytest driver keeps it honest.

What changes are included in this PR?

• examples/datafusion-ffi-example/run_demo.py: Adds a standalone script that walks the conformance matrix in numbered sections (table provider, functions, catalog provider, config extension, and codec round-trip). Includes a strict import guard that instructs users to run maturin develop if the extension is not built.
• examples/datafusion-ffi-query-planner-example/run_demo.py: Adds a standalone script that demonstrates and prints the logical plan, physical plan, the effect of SET ffi_query_planner.max_rows, and planner nesting. Includes the same import guard.
• python/tests/_test_run_demo.py (in both crates): Adds driver tests that execute the demo scripts natively via subprocess.run([sys.executable, script]) and assert on their output anchor lines to guarantee they continue running standalone.
• README.md (in both crates): Adds a concise three-line header explaining what the directory is, linking to the protocol guide, and showing exactly how to run the demo.

Are there any user-facing changes?

Yes, this adds fully standalone, runnable Python examples that users can execute directly to learn the FFI bindings without dealing with the internal test suite.