test(keyword-clusters): cover SEO keyword helpers

[terminalchai]

What does this PR do?

Adds co-located Vitest coverage for titleCaseKeyword, fixKeywordInitialisms, and getSampleKeywordTerms.

The tests cover initialism and certification-code casing, function-word title casing, generic-token filtering, minimum token length, duplicate removal, and stable ordering.

Why?

These helpers generate locked SEO text across domain landing pages but previously had no direct unit coverage.

Closes #72

How was this tested?

• npm run check - passed, 307 tests
• npm run build - passed with all post-build guards
• npm run lint - passed
• npm run validate - passed with existing SAA-C03 warnings
• Focused test - 7 passed
• Isolated E2E run - 103 passed, 20 skipped, 6 unrelated existing failures in analytics, auth-banner, dashboard, and contrast flows

Checklist

• npm run build passes locally
• npm run lint passes locally
• npm run test passes locally
• npm run validate passes locally
• No new third-party dependencies

Screenshots (UI changes only)

Not applicable. This is a tests-only change.

Implementation and verification assisted by OpenAI Codex.