test: add coverage for GridGenerator.ring and .spiral

[dusan-maintains]

Summary

• Add focused test coverage for GridGenerator.ring() and GridGenerator.spiral().
• Validate distance/radius invariants and uniqueness for ring output.
• Validate center inclusion and expected cell count for spiral output.

Why

• Addresses missing coverage highlighted in issue Test coverage for the Ring and Spiral static methods in GridGenerator.tsx #87.
• Improves confidence around two methods currently marked as potentially untested in source comments.

Test Plan

• npx jest test/src/GridGenerator.test.tsx --runInBand
• Result in this environment: all 9 tests passed.

Notes

• Scope is intentionally limited to tests only; no runtime behavior changes.