[CONTRACT] Implement Multi-Token Escrow Support

[dinahmaccodes]

Description

Extend the escrow system to support multiple token contracts per shipment.
Different shipments can hold escrow in different SAC or custom token contracts.

Tasks

• Add token_address: Address field to Shipment struct
• Add token_address parameter to create_shipment() (required)
• Update escrow storage: key pattern (Symbol("shipment_token"), shipment_id) → Address
• Update deposit_escrow() to use per-shipment token address
• Update release_escrow() and refund_escrow() to use per-shipment token
• Add get_shipment_token(shipment_id) → Result<Address, NavinError> query
• Emit token_address in: escrow_deposited, escrow_released, escrow_refunded events

Acceptance Criteria

• Different shipments hold escrow in different token contracts independently
• Token address stored immutably per shipment, used consistently in all escrow ops
• Escrow operations reject if token_address doesn't match shipment record
• Unit tests:
  • Single shipment, single token
  • Two shipments, two different tokens, verify independent balances
  • Cross-token rejection (attempt wrong token on shipment)
  • get_shipment_token() returns correct address per shipment

PR Checklist

• Shipment struct extended with token_address
• Storage key pattern documented
• All escrow functions updated
• Event signatures updated (all 3 escrow events emit token address)
• 4+ unit tests passing
• All CI checks pass

[m1s0g1]

@m1s0g1 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hi maintainer, I want to work on smart contracts and would love to contribute. Can I be assigned this issue?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @m1s0g1 to this issue.

[drips-wave]

Congratulations, @m1s0g1! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @m1s0g1: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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