feat: Solidity escrow contract implementation

[teethaking]

Implement the core GrantStream escrow contract in Solidity.
Contract should handle grant creation, fund locking, milestone
submission, and USDC release on approval. Deployable on Base Sepolia.

[Dharmesh070294]

Hi, I’d like to work on this issue. I’ll add a minimal Hardhat setup and implement the core GrantStreamEscrow contract with USDC funding, milestone submission using IPFS evidence links, verifier approval, and milestone-based fund release for Base Sepolia.