Add staking contract example

[alexgduarte]

Contributes to #11

Summary

• Add an original staking smart contract example.
• Include stake, unstake, staked-balance query behavior, messages, state, and unit coverage.
• Add a schema binary target, remove an unresolved helper-module export, and reject malformed unstake amounts instead of treating them as zero.

Validation

• git diff --check

Rust validation was not run in this Windows workspace because cargo and rustc are not installed on PATH.

Bounty Contact

GitHub: @alexgduarte
Email: alexduartegomescouto@gmail.com