Add Stream Milestone Vesting Support

[grantfox-oss]

Type: Feature | Priority: LOW | Effort: 6 hours
Component: contracts/Contract-V1/src/lib.rs

Problem:
Only cliff period vesting supported. Need milestone-based vesting.

Acceptance Criteria:

• Create MilestoneVesting structure
• Support up to 10 milestones per stream
• Validate milestones ordered by time
• Calculate vested amount at any point
• Test with 4 milestones
• Document in README

[gloskull]

Hello, I have 4 years of experience delivering solutions for projects like this, covering backend, frontend, and smart contract development. After carefully reviewing your requirements, I have a clear understanding of the task and am confident in my ability to deliver a clean, efficient, and thoroughly tested solution on time. I prioritize code quality, maintainability, and industry best practices throughout the development process.
My PR will be submitted in under 10hours.

[Silver36-ship-it]

I would love to work on this professionally, please assign it to me

[Tobi-8]

Hey maintainer, I have experience in handling this issue
Kindly assign

[mijinummi]

Hello maintainers I'm a full stack developer experience in building scalable systems I can handle this perfectly if assigned

[grantfox-oss]

🦊 GrantFox — @mijinummi has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #1190)
2. Your PR will be reviewed by the StellarStream-HQ maintainers

Good luck! Track your progress on GrantFox.