feat: Implement Client-Side Transaction Builder

[KingFRANKHOOD]

Implement client-side Stellar donation transaction builder (XDR-ready) Closes #27

Description

This PR adds a donation transaction builder service to construct Stellar payment transactions for frontend signing workflows. It supports native and issued assets, adds project memo tagging, calculates transaction fees, and returns signable XDR with robust error handling.

What Changed

• New Service: Added a dedicated donation transaction builder in donation_tx_builder.rs.
• CLI Integration: Added build-donation-tx command in main.rs to generate unsigned XDR.
• Destination Resolution: Implemented destination resolution from environment variables (STELLARAID_PLATFORM_PUBLIC_KEY) with CLI override support.
• Validation & Error Handling:
  • Invalid donor/destination addresses and timeout validation.
  • Invalid/negative/over-precision amounts.
  • Asset validation (including missing issuer for non-XLM assets).
  • Memo validation (ASCII + max 28 bytes).
• Logging: Added non-sensitive transaction logging (destination, asset, amount, memo, fee).
• Tests: Added coverage for XLM, 4-char (USDC), and 12-char assets, memo presence, and error paths.

Transaction Structure

The generated transaction envelope follows this schema:

• Source: Donor address.
• Operations: Single Payment op (Destination: platform address).
• Asset: Selected asset (XLM or issued).
• Memo: project_<project_id>.
• Fee/Timeout: Configured base fee and transaction timeout.
• Output: Unsigned XDR ready for wallet/client-side signing.

Acceptance Criteria Mapping

• Transactions build successfully for all supported asset types.
• Memo correctly includes project ID.
• Fee is correctly applied.
• XDR is valid and ready for signing.
• Errors are caught and surfaced clearly.

🛠 Validation performed

• Ran targeted tests for tools transaction builder logic.
• Ran workspace checks via make ci (audit/deny/fmt/lint/tests) successfully.

[KingFRANKHOOD]

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