docs(example): quarantine foreign plan workaround

[akashchamp]

Which issue does this PR close?

Closes #1725

Rationale for this change

The FFI example needs a temporary process-local registry to preserve an opaque
ForeignExecutionPlan wrapper. Keeping that workaround beside the general
physical codec makes its broad claim and non-portable payload look reusable.

What changes are included in this PR?

• Move the temporary execution-plan token registry into a dedicated
  foreign_plan_workaround module.
• Mark the module as not a reusable pattern, link the blocking upstream issue,
  and record the removal conditions.
• Keep the physical codec's existing behavior by delegating its claim, park,
  and take operations to the isolated module.

How are these changes tested?

• cargo clippy --no-deps --all-targets
• Focused physical extension-codec test suite: 4 passed.
• Manually built the FFI example, installed it into the project environment,
  then serialized and restored a physical plan through MyPhysicalExtensionCodec.
• Repository all-files pre-commit was attempted: ruff, clippy, codespell, and
  lock validation passed. The workflow hook could not run because the remote
  host has no Docker daemon, and the project's nightly rustfmt hook could not
  be installed because the Rust distribution endpoint timed out. The stable
  cargo fmt --all -- --check command passed.

No new test is needed because this keeps the codec behavior intact; the focused
test and the manual round-trip exercise the delegated encode/decode path.