Add quiz contract example

[alexgduarte]

Contributes to #11

Summary

• Add a new quiz simple contract example.
• Let the owner create multiple-choice quizzes with validated title, question, and options.
• Support optional closing height, manual close, one answer per player per quiz, correctness recording, player stats, and paginated quiz listing.
• Include README documentation, root README navigation, schema generation, and eight focused unit tests.

Validation

• cargo fmt --manifest-path quiz/Cargo.toml -- --check
• cargo test --manifest-path quiz/Cargo.toml --target-dir /private/tmp/codex-quiz-target
• git diff --check