[Smart Contract] Implement Soroban Shipment Contract

[mftee]

Description:
Create the on-chain shipment tracking contract that mirrors the off-chain shipment lifecycle. This provides an immutable, decentralized audit trail for each shipment's status changes.

Create:

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

Acceptance Criteria:

• ShipmentStatus enum: Created, Accepted, InTransit, Delivered, Completed, Disputed, Cancelled
• Shipment struct with #[contracttype]: id: u64, shipper: Address, carrier: Option<Address>, status: ShipmentStatus, created_at: u64, updated_at: u64
• DataKey: Shipment(u64), Counter, ShipperList(Address), CarrierList(Address)
• initialize(env, admin) — admin setup
• create_shipment(env, shipper: Address) — shipper.require_auth(), returns new shipment ID, appends to shipper list
• accept_shipment(env, shipment_id, carrier: Address) — carrier.require_auth(), sets carrier, Created → Accepted, appends to carrier list
• mark_in_transit(env, shipment_id, carrier: Address) — carrier only, Accepted → InTransit
• mark_delivered(env, shipment_id, carrier: Address) — carrier only, InTransit → Delivered
• confirm_delivery(env, shipment_id, shipper: Address) — shipper only, Delivered → Completed
• cancel_shipment(env, shipment_id, caller: Address) — caller must be shipper or carrier, only from Created or Accepted
• raise_dispute(env, shipment_id, caller: Address) — party to shipment, from InTransit or Delivered
• resolve_dispute(env, shipment_id, admin: Address, complete: bool) — admin only
• get_shipment(env, id), get_shipper_shipments(env, shipper), get_carrier_shipments(env, carrier)
• All shipments extend TTL ~1 year on write
• 10 unit tests covering happy path + error cases

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[AbdulmujibOladayo]

@AbdulmujibOladayo has applied to work on this issue as part of the Stellar Wave Program's 2nd wave.

I am a fullstack software developer with multiple years experience in the industry. I can deliver on this issue in a few hours.

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Congratulations, @AbdulmujibOladayo! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on February 26, 2026.

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This issue has been completed by @AbdulmujibOladayo as part of the Stellar Wave Program's 2nd Wave 🥳

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