SC: Implement Yield Distribution Smart Contract

Description: Build a Soroban contract for YieldForge to calculate and distribute yields (e.g., from trading fees or lending interest) to staked users based on their pool share.

Tasks:

Write a Rust contract to track pool performance and distribute yields.
Implement yield calculation (e.g., proportional to stake, fixed 5% APY for testing).
Test distribution with sample data (e.g., 100 USDC stake).

Acceptance Criteria:

Contract calculates yields based on user stake and pool performance.
Yields are distributed to users on testnet (e.g., via stablecoin payments).
Distribution events are verifiable via Horizon API.

No source files, tests, or entry points are named. Start by locating the Soroban contract structure and the testnet payment and Horizon integration points, then define how stake, pool performance, and the fixed test APY are represented. Done means sample distributions work, yields are paid to staked users on testnet, and the resulting events can be verified through Horizon API.

Written by the indexing model from the issue text.