feat: add soroban-sdk-nexus security, interop, and tooling modules

[Clawue884]

What

This PR introduces the initial version of soroban-sdk-nexus, an extended structure on top of rs-soroban-sdk focused on:

• Secure smart contract patterns (access control, reentrancy guard, pausable)
• Interoperability layer (message format, signature verification, bridge events)
• Developer tooling (gas analyzer & contract risk scoring)
• Educational examples & tutorials

Why

The goal is to provide a practical, production-oriented enhancement for Soroban developers who want:

• Better security-by-design patterns
• Cross-network / bridge-ready primitives
• Tooling for contract analysis
• Structured examples for DeFi, DAO, and dApps

This helps lower the barrier for building secure and interoperable applications on Soroban.

Known limitations

• Examples are minimal and not yet production audited
• Interop layer currently focuses on event-based bridge patterns
• Tooling is heuristic-based (not full static analysis yet)

[Clawue884]

Hi Stellar team 👋
This is my first contribution here.
I’ve added an initial structure called soroban-sdk-nexus with security patterns, interop helpers, tooling, and tutorials.

Happy to revise anything to better fit the project direction.
Thanks for reviewing 🙏

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