feat: implement milestone-based release schedules in escrow contract

[Cjay-Cyber-2]

Description — what to implement and the why/impact.

Implement milestone-based release schedules in the escrow contract (contracts/escrow). Currently, escrows typically release funds based strictly on time-locks. Adding milestone-based tracking allows funds to be released incrementally as specific project phases or deliverables are verified and approved on-chain by authorized reviewers.

Context & Requirements — background, constraints, design references, edge cases;

• Grounded in contracts/escrow using soroban-sdk.
• Must allow defining an array or ordered list of milestones, each with a specific fraction or amount of the escrowed funds and a status (Pending, Completed, Disputed).
• Must allow authorized reviewers to mark milestones as completed and trigger partial payouts.
• Must handle edge cases such as partial withdrawals and cancellation of remaining unreleased funds.

Acceptance Criteria — a checklist ("- [ ] ...") of specific, testable conditions that define "done".

• Define milestone data structures and storage keys in contracts/escrow.
• Implement functions to initialize milestone schedules, approve individual milestones, and release corresponding funds.
• Emit deterministic error codes for unauthorized milestone approvals or invalid milestone states (INVALID_MILESTONE, MILESTONE_ALREADY_COMPLETED).
• Add comprehensive unit tests covering multi-milestone creation, sequential completion, and partial fund release.

Implementation Guidance — likely files/modules to touch and a suggested approach

• Likely files: contracts/escrow/src/lib.rs, contracts/escrow/src/storage.rs, and associated tests.
• Approach: Store milestones in a vector or indexed map in persistent storage. Ensure checked arithmetic is used when calculating payout amounts per milestone to prevent rounding errors or fund leakage.

Testing & Validation — how the contributor should prove it works

• Run cargo test --package astroid-escrow to verify all escrow and milestone test cases.
• Build the optimized WASM via stellar contract build to ensure compilation succeeds.

Submission Guidelines — must open a PR that includes "Closes #"; assignment is required before starting; follow the repo's existing style and conventions.

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Wave complexity: Medium

[Luchistack]

@Luchistack has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer I would love to work on this issue.

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

[Riel101]

@Riel101 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi, i cantackle this for you

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Riel101 to this issue.

[drips-wave]

Congratulations, @Riel101! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @Riel101: 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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