feat: add creator and protocol fee split logic with tests

[wheval]

Summary

Closes #4

• Add fee module with FeeConfig (creator_bps, protocol_bps) and compute_fee_split for creator/protocol fee calculation
• Add DataKey::FeeConfig, set_fee_config, get_fee_config, and compute_fees_for_payment to creator-keys contract
• Add unit tests for fee split math and integration tests for contract methods
• Add docs/fee-assumptions.md documenting rounding rules, validation, overflow limits, and client/server integration notes

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

Checklist

• Linked issue or backlog item
• Contract behavior and invariants are described clearly
• Docs updated if contract interfaces or workflows changed
• Scope stays limited to one contract concern

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