docs: add Squarespace CSV fallback fixture

[RujulaSharma]

Summary

• Add a synthetic Squarespace-like order CSV fixture.
• Add invalid order examples covering documented validation cases.
• Document required and optional columns, privacy boundaries, and CSV/API usage.
• Add fixture tests covering the documented schema and integration boundary.

Testing

• node --test tests/squarespace-csv-fallback-fixture.test.js
• 4 tests passed

Scope

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

It does not implement a Squarespace connector, live API access, storefront scraping, or real merchant-data ingestion.

Closes #425

[RujulaSharma]

The fixture tests pass locally (4/4).

The Packaging checks job is currently failing in the existing
check-advisories.js step because of 6 unreviewed brace-expansion
production advisories. This PR does not modify dependencies or the
package lockfile, so I have kept the change scoped to issue #425.

[sridharkalaibala]

Thanks for the focused, synthetic fixture and boundary documentation. I verified the diff is limited to fixtures, docs, and tests, and node --test tests/squarespace-csv-fallback-fixture.test.js 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 advisory check is green, then we can complete review.

[sridharkalaibala]

Integrated into develop through #1099 with the original commit preserved. Closing this main-targeted duplicate; promotion to main is tracked by #1094.