[Hollujay]

Summary

• Add a human-readable decoder case specification.
• Build real Stellar transactions from the shared case definitions using the installed Stellar SDK.
• Expand decoder test coverage from 4 operation types to 11.
• Exercise classic operations plus a Soroban invokeHostFunction fallback case.
• Keep the production decoder unchanged.

Testing

• npm test — 55/55 passed
• npm run typecheck — passed
• npm run lint — passed
• git diff --check — clean

Scope

Test-only change.
No production decoder changes.
No dependency changes.
No CI changes.
No raw XDR fixtures.