ocument SDK transaction lifecycle architecture

[Johnsource-hub]

#closes
#175

Summary
This PR adds a comprehensive Architecture Decision Record (ADR) detailing the PocketPay SDK’s transaction lifecycle architecture and safety assumptions. It also updates the main README.md to reference this new architectural document.

Establishing these boundaries prevents architectural drift, ensures transaction helpers evolve consistently, and provides consumers with a clear reference for error handling, retries, polling, and security responsibilities.

Key Changes
New Architecture Decision Record (docs/adr/001-transaction-lifecycle-architecture.md):

Lifecycle Stages: Documents the complete flow from preparation, client-side signing, and RPC/Horizon submission, to post-submission status verification.

Retry & Polling Strategy: Specifies explicit guidelines for safe vs. unsafe retries (e.g., handling transient network errors vs. replay prevention) and polling logic for transaction finality.

Security Boundaries: Clarifies that the SDK operates on a zero-persistence model for secret keys, emphasizing consumer responsibility for key storage, user authentication, and transaction approval.

Documentation Link Update (README.md):

Updated the Architecture Decision Records link in the README.md to point directly to the newly created ADR.

[El-swaggerito]

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