Add architectural documentation for Pi Ruby SDK

[Clawue884]

Summary

This PR adds a new documentation file: docs/architecture-notes.md.

The document provides architectural notes and design context for the Pi Ruby SDK,
helping developers understand system boundaries, responsibilities, and intended
usage patterns.

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Motivation

While the SDK API is well-documented, there is limited high-level explanation
of why the payment flow is structured as it is and where the SDK fits
within the broader Pi Network architecture.

This document aims to improve developer understanding without introducing
new functionality or changing existing behavior.

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Scope of Changes

• Adds docs/architecture-notes.md
• No code changes
• No behavioral or API changes

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Key Topics Covered

• Architectural positioning of the SDK
• Hybrid off-chain / on-chain payment model
• Payment lifecycle and state management
• Idempotency and backend safety considerations
• Explicit non-goals and design constraints

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Notes

This document is descriptive only and does not represent an official roadmap
or future commitments. All content aligns with existing SDK behavior and
current Pi Network application guidelines.