Implement sender stream indexing

[Jagadeeshftw]

Description
Add sender-based stream index to support efficient listing and analytics queries.

Requirements and context

• Must be secure, tested, and documented.
• Must remain consistent through all lifecycle operations.

Suggested execution

• Branch: git checkout -b feature/sender-stream-index
• Add sender-index storage updates on creation and lifecycle changes.
• Add tests for index consistency and ordering.
• Document pagination expectations.

Example commit message
feat: implement sender stream index

Guidelines

• Minimum 95 percent test coverage.
• Clear documentation.
• Timeframe: 120 hours.

[Emmanuelluxury]

I’d like to work on this issue because it sits at the intersection of system design, financial logic, and secure contract architecture — areas I actively specialize in and enjoy building within. The problem requires careful handling of state transitions, precision math, and security constraints, which makes it both technically meaningful and impactful to the overall reliability of the protocol.

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[emarc99]

@emarc99 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Would like to do this!

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[Fayvor22]

@Fayvor22 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I would love to work on this and get it resolved

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[magaret457]

@magaret457 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hi maintainer. I can work on this issue, please assign.

Thank you!

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[Pvsaint]

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

I'm a software dev, may I take on this issue if it's still available?
ETA 2hrs

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