[Backlog Wave 6][Contracts] Add contract interface diff examples for reviewer guidance

[Godsmiracle001]

Area: Contracts
Wave: Backlog Wave 6

Problem:
Add contract interface diff examples for reviewer guidance is a tracked follow-up for hardening SplitNaira across backend, frontend, contracts, CI, docs, ops, and security surfaces.

Scope:

• Confirm the current behavior and owner for this subsystem.
• Add or update implementation, tests, or documentation as appropriate.
• Keep changes focused and compatible with the existing repo patterns.

Acceptance criteria:

• The behavior or documentation gap is addressed.
• Relevant local checks pass for the touched workspace.
• Any user-facing or operational impact is documented.

---

Created from requested repository backlog expansion.