[Epic] SEO testing utilities, snapshot serializers, and generated schema types

[lahiruudayakumara]

Objective

Let consumers test SEO behavior without brittle selectors and provide stronger structured-data types.

Scope

• Matchers: toHaveCanonical, toBeIndexable, toHaveValidStructuredData, toHaveHreflang, and related assertions.
• Stable snapshot serializer grouping tags and normalizing irrelevant ordering.
• SSR assertion helpers.
• Reproducibly generated Schema.org foundations with Google overlays.
• Discriminated unions for supported rich-result types.
• Vitest and Jest support.

Acceptance criteria

• Optional testing entry point/package
• DOM, Helmet state, and SSR string support
• Actionable assertion failures
• Snapshot versioning policy
• Pinned/reproducible type generation
• Type-check and bundle performance review
• Vitest and Jest examples.