[Smart Contract] Implement Soroban Escrow Contract

[mftee]

Description:
Create the on-chain payment escrow contract. When a shipment is funded, tokens are locked in the contract. They are released to the carrier on confirmed delivery, or refunded to the shipper on cancellation. Uses the SEP-41 token interface (token::Client) for actual token transfers.

Create:

• contracts/escrow/Cargo.toml
• contracts/escrow/src/lib.rs

Acceptance Criteria:

• EscrowError: AlreadyFunded, NotFunded, Unauthorized, NotInitialized, InvalidState, AlreadyDisputed
• EscrowStatus: Unfunded, Funded, Released, Refunded, Disputed
• EscrowRecord struct: shipment_id: u64, shipper: Address, carrier: Option<Address>, token: Address, amount: i128, status: EscrowStatus
• DataKey: Escrow(u64), Admin
• initialize(env, admin)
• fund_escrow(env, shipment_id, shipper, carrier, token, amount) — shipper.require_auth(), calls token::Client.transfer_from(contract, shipper, contract, amount) (requires prior approve), stores record
• release_payment(env, shipment_id, caller) — only admin or shipper; status must be Funded; calls token::Client.transfer(contract, carrier, amount); sets status Released
• refund_payment(env, shipment_id, caller) — only admin; status must be Funded; refunds to shipper; sets status Refunded
• raise_dispute(env, shipment_id, caller) — shipper or carrier, from Funded only
• resolve_dispute(env, shipment_id, admin, release: bool) — admin only from Disputed; releases or refunds
• get_escrow(env, shipment_id)
• TTL extended on all write operations
• 8 unit tests including full happy path using StellarAssetClient for test token setup