Add a two-cell boundary example with passing and failing checks

[aoto-tech]

Problem

CellFence has a minimal single-cell example and many fixtures, but there is no small example dedicated to the core two-cell boundary story: a valid public import passes, while a private implementation import fails.

That makes the README explanation harder to turn into a local, copy-pasteable experiment.

Expected behavior

Add a small two-cell example that demonstrates both paths:

• reporting imports parser through the declared public entry and passes.
• reporting imports parser/internal/... and fails with CELLFENCE_PRIVATE_IMPORT.

Scope

Keep this example small and focused on import boundaries.

Do not add baseline signing, waivers, resource adapters, or benchmark logic to this example.
Do not change CellFence rule behavior.

Suggested starting points

Likely areas:

examples/
tests/examples.test.mjs
README.md

Existing related fixtures:

fixtures/valid/public-import
fixtures/invalid/private-cross-cell-import

Acceptance criteria

• Add a documented example under examples/ with parser and reporting cells.
• The example has one command that shows the valid case passing.
• The example has one command that shows the private import case failing with CELLFENCE_PRIVATE_IMPORT.
• The example is linked from the README or examples section.
• The example test coverage is updated so the passing case is exercised automatically.
• No generated benchmark or research output is added.

Validation

Run:

npm run build
node --test tests/examples.test.mjs
npm test

If only the example test is run locally, mention that in the PR.