docs: add end-to-end integration walkthrough for embedding the SDK

[karagozemin]

Summary

Expand docs/INTEGRATION.md with a complete, copy-paste walkthrough covering install → create round → commit sealed bid → open reveal → settle → export receipt.

Motivation

README.md promises reusable infrastructure, but there is no single continuous example a partner team can follow start to finish.

Acceptance criteria

• Numbered walkthrough with runnable snippets.
• Notes on required env / keys and testnet vs mainnet.
• Link-checked (scripts/check-links.js).
• Cross-linked from README.

[benzy018]

Good day, Maintainer. I'll validate the workflow from a fresh setup before submitting a clean pull request. I would appreciate being assigned this issue.

[blvckmamba81-sketch]

Hi! I'd like to work on this issue. I have experience writing clear technical documentation and can create a concise end-to-end integration walkthrough that makes embedding the SDK straightforward for developers. Thank you!