[CT-11] Add platform fee deduction to escrow contract

[mftee]

Problem

The escrow contract releases 100% of the held funds to the carrier on delivery confirmation. There is no mechanism to deduct a platform service fee before release. Without this, the platform has no on-chain revenue model and must rely entirely on off-chain accounting.

Proposed Solution

Create a fee-aware escrow contract inside contracts/package/escrow-with-fees/ that automatically deducts a configurable platform fee from the release amount and routes it to a treasury address.

Note: All work must be done inside the contracts/package/ directory. Do not modify any existing contract files outside this folder.

Acceptance Criteria

• platform_fee_bps: u32 (basis points) set at contract initialization (e.g., 250 = 2.5%)
• Fee is deducted from the escrow amount on release_funds, not on fund_escrow
• Fee is transferred to the platform_treasury: Address set at initialization
• Carrier receives funded_amount - fee; treasury receives fee
• Full refunds (on cancellation) return the full funded amount to the shipper — no fee on refunds
• A ReleaseWithFee event emits { carrier_amount, fee_amount, treasury_address }
• platform_fee_bps is updatable by admin; an update event is emitted on change
• Unit tests verify fee math for 1%, 2.5%, and 0% fee rates

[a-malik-gh]

@a-malik-gh has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I’d love to work on this task and contribute a solution for it.
I’ll review the requirements carefully and submit an implementation/update as soon as possible.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @a-malik-gh to this issue.

[drips-wave]

Congratulations, @a-malik-gh! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @a-malik-gh: 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.

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