Add food truck catering social example

[mintyPT]

Summary

• Adds a manifest-driven food truck and catering social post example set with three variants: route preorders, office catering inquiries, and event bookings.
• Uses source-neutral Unsplash photos passed through variant JSON layers, plus rendered outputs and a contact sheet.
• Updates README documentation and Unsplash attribution for the new example set.

Closes #23.

Verification

• npm run examples:render -- examples/use-cases/food-truck-catering.manifest.json
• npm run examples:contact-sheet -- examples/use-cases/food-truck-catering.manifest.json examples/use-cases/food-truck-catering-contact-sheet.png
• npx vitest run test/food-truck-catering-example.test.ts
• npm run check
• npm run build

Note: local full npm test was attempted, but existing CLI/browser tests timed out under the shared local environment; CI should provide the clean runner result.