Pull Request: Implement Creator Co-Op Split (Multi-Sig Revenue Sharing) (#56, #15)

[JerryIdoko]

📝 Description
This PR introduces the Splitter Logic to the SubStream Protocol, allowing a single subscription revenue stream to be automatically divided among multiple contributors (up to 5 addresses). This enables "Co-Op" content creation where Editors, Guests, and Lead Creators are paid instantly and transparently.To prevent the common "dust" problem in DeFi, I have implemented Fixed-Point Arithmetic using a scaling factor of $10^7$, ensuring that even the smallest fractions of a cent are accounted for during the division process.

🎯 Key Changes
SplitConfig Structure: Added a state object that maps up to 5 Address keys to their respective percentage (represented as basis points).

High-Precision Distribution: Implemented the distribute_revenue function using a "remainder-retention" strategy to ensure the total sum of distributed tokens always matches the initial balance.

Gas-Efficient Storage: Used a Vec for the share-holders to minimize the footprint on the Soroban ledger.

Safety Checks: Added validation to ensure the total sum of percentages in a SplitConfig exactly equals 100% (10,000 basis points).

💻 Implementation
Detail: The Split LogicThe contract calculates each share as follows:$$\text{Share Amount} = \frac{\text{Total Balance} \times \text{Basis Points}}{10,000}$$Any remaining "dust" from the division is automatically allocated to the Lead Creator (the first address in the config) to ensure the contract balance returns to zero after withdrawal.

✅ Acceptance Criteria Checklist
[x] Multi-Sig Support: Verified that revenue can be split between 2 to 5 distinct addresses.

[x] Fixed-Point Accuracy: Unit tests confirm that zero tokens are "stuck" in the contract after a full distribution of 1,000,000,001 stroops.

[x] Admin Controls: Only the stream owner can modify the SplitConfig before the first payment is received.

[x] Withdrawal Flow: Each participant can independently trigger the withdraw to claim their specific share.

🚀 How to Verify
Test the Splitter:

Bash
cargo test test_split_precision
Verify Boundary Conditions: Run the test suite with 5 addresses and an odd number of tokens to ensure the remainder logic works correctly.

🔗 Linked Issues
Closes #56
Closes #15

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