docs: add OpenCart option price adjustment fixture

[RujulaSharma]

Summary

• Add synthetic OpenCart product and order fixtures for option price adjustments.
• Cover positive, negative, and zero option price deltas.
• Document expected POS line-item mappings for quantity, price, discount, tax, and totals.
• Add unsupported option combination fixtures.
• Reference official OpenCart documentation.
• Add deterministic tests for the fixture behavior.

Testing

• node --test tests/opencart-option-price-fixture.test.js
• 4 tests passed

Scope

This PR is limited to synthetic fixture data, documentation, and tests.

It does not implement an OpenCart API client, live store integration,
credentials, scraping, or real merchant-data ingestion.

Closes #426

[sridharkalaibala]

Thanks for the focused, synthetic OpenCart option-pricing fixture and boundary documentation. I verified the focused test passes (4/4). I am holding approval because the current merge result has a failing Packaging checks job. Please rebase or rerun once that repository-level check is green, then we can complete review.