Implement AnchorKit escrow milestone validator

[El-swaggerito]

Description

Difficulty: Expert

Type: Feature

Summary

Implement a validator for escrow milestone data.

Current Behaviour

Escrow milestone examples may not consistently validate title, amount, status, evidence, release signer, and ordering.

Expected Behaviour

AnchorKit should provide shared validation for milestone-based escrow flows.

Suggested Implementation

Create milestone types, validation helpers, fixtures, tests, examples, and docs.

Files or Areas Likely Affected

• packages/escrow/
• packages/validators/
• packages/types/
• examples/
• tests/
• docs/

Acceptance Criteria

• Escrow milestone validator is implemented.
• Milestone title, amount, status, evidence, and signer fields are validated.
• Invalid milestones return typed errors.
• Examples use the validator.
• Tests cover valid and invalid milestones.
• Docs explain validation rules.

Additional Notes

This is suitable for GrantFox contributors because it requires implementation and tests.