feat: introduce modular production-grade architecture for Pi SDK JS

[Clawue884]

🚀 Summary

This PR upgrades pi-sdk-js from a thin browser wrapper into a modular, production-grade SDK architecture suitable for real-world Pi applications such as marketplaces, SaaS platforms, and SuperApps.

The refactor preserves full compatibility with the official Pi Browser SDK while significantly improving maintainability, extensibility, and developer experience.

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🧠 Key Improvements

• ✅ Modular architecture (separation of concerns)
• ✅ Clear SDK core vs feature modules
• ✅ Typed payment lifecycle engine
• ✅ Event-driven design (payment & user events)
• ✅ Marketplace-ready structure (merchant registry)
• ✅ Plugin system foundation
• ✅ Audit trail for payments
• ✅ Mock-friendly & production-safe design

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🗂️ New Structure

src/ ├─ core/ # config, event bus, payment engine ├─ auth/ # Pi authentication ├─ payment/ # payment creation & lifecycle ├─ marketplace/ # merchant abstraction ├─ plugins/ # SDK extension system ├─ audit/ # audit & traceability ├─ types/ # shared TypeScript types ├─ sdk.ts # main SDK class └─ index.ts # public exports

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🔄 Backward Compatibility

• No breaking changes to the underlying Pi Browser SDK usage
• Still relies on window.Pi as the source of truth
• Designed as an application-layer SDK, not a consensus or ledger abstraction

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🎯 Motivation

This refactor enables:

• Cleaner long-term maintenance
• Easier community contribution
• Advanced use cases (marketplace, escrow, plugins, analytics)
• Alignment with professional SDK standards (Stripe-like architecture)

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🧩 Next Possible Extensions (Out of Scope)

• Server-side payment verification helpers
• Escrow & split-payment logic
• Fraud / analytics plugins
• Official marketplace examples

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Status: Ready for review
Impact: Structural (non-breaking, additive)
Audience: SDK & application developers