Create Reward System Module

[Mkalbani]

Description

Implement a rewards module that manages stella-based reward distribution tied to puzzle completion.

Acceptance Criteria

• A rewards module exists under src/rewards/mod.rs
• Reward struct contains type (stella), amount, and condition trigger
• Rewards are granted only when puzzle solve conditions are met
• Duplicate reward prevention logic is in place
• Unit tests cover reward grant and duplicate prevention

[samuelfrancis163-eng]

i am confident i can work on this issue. Please assign it to me

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #4)
2. Your PR will be reviewed by the MindFlowInteractive maintainers

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