Feat/co creator fee unit tests

[Unclebaffa]

✅ I've successfully implemented comprehensive unit tests for the co-creator fee split invariant. Here's what was delivered:

📋 Acceptance Criteria - All Met

✅ Co-creator amount + creator recipient amount = total creator fee
✅ No XLM lost at 30%, 50%, and 10% share values
✅ Tests fail if either party receives more than their share

📝 Test File Created

creator-keys/tests/co_creator_fee_split_invariant.rs (317 lines)

Contains 8 comprehensive tests:

1. test_co_creator_fee_split_30_percent_no_xlm_lost - Core requirement: 30% split
2. test_co_creator_fee_split_50_percent_no_xlm_lost - Core requirement: 50% split
3. test_co_creator_fee_split_10_percent_no_xlm_lost - Core requirement: 10% split
4. test_co_creator_fee_split_invariant_across_multiple_trades - 5 consecutive trades
5. test_co_creator_fee_split_invariant_on_sell - Sell operations
6. test_co_creator_fee_split_boundary_cases - Min (0.01%) and max (99.99%) shares
7. test_no_party_receives_more_than_their_share - 10%, 30%, 50%, 70%, 90% validation
8. test_co_creator_fee_split_with_odd_price_amounts - Rounding edge cases

📦 Commits Made

249446d Add comprehensive test implementation documentation
c141f59 Add edge case test for odd price amounts with fractional percentages
30bc12e Add co-creator fee split invariant test with 30%, 50%, and 10% shares

✅ 3 commits

🔍 Key Verification Points

Each test verifies:

• Balance increases before/after trades
• Sum of both parties equals total creator fee
• Individual shares match expected calculations
• No XLM lost due to rounding
• Neither party receives more than allocated

The implementation is complete and ready for CI/CD validation! 🎉

Closes #545