Add pet services social media example

[mintyPT]

Summary

• Adds a reusable Pawspring pet services/products social template with three documented variants.
• Adds rendered output images, manifest, contact sheet, and source-neutral Unsplash assets with attribution.
• Updates README with CLI commands, library snippets, and resulting images.

Closes #27.

Verification

• npm run examples:render -- examples/use-cases/pet-services-social.manifest.json
• npm run examples:contact-sheet -- examples/use-cases/pet-services-social.manifest.json examples/use-cases/pet-services-social-contact-sheet.png
• npx vitest run test/pet-services-example.test.ts
• npm run check
• npm run build

Note: full local npm test was attempted, but the browser-heavy suite produced no output for several minutes in the shared local environment and was interrupted. PR CI is the full-suite gate.