Add leaderboard contract example

[alexgduarte]

Contributes to #11

Summary

• Add a new leaderboard simple contract example.
• Track one best-score entry per player with self-service submission and owner-administered score management.
• Reject lower score updates, support score removal, configurable display-name and metadata limits, and top-score/player queries.
• Include README documentation, root README navigation, schema generation, and seven focused unit tests.

Validation

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