Add storage key prefix isolation between contracts

[Samuel1-ona]

Ensure storage key prefixes cannot collide between the three contracts.

Acceptance Criteria:

• Document all storage key prefixes
• Verify no overlap between contracts
• Add prefix constants
• Test key isolation

[drips-wave]

Congratulations, @solomonokeke012-bot! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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