Document the three lending test modules' purpose

Problem Statement

contracts/lending/src/lib.rs contains three mod tests blocks plus test.rs; new contributors can't tell which to extend.

Why it matters

Discoverability keeps test coverage from rotting.

Technical Context

Add a doc-comment for each mod tests block describing intent.

Expected Outcome

Each block carries a 2-line rustdoc header.

Acceptance Criteria

1. cargo doc --no-deps renders descriptions.
2. PR template points to the right file.
3. README mentions layout.

Implementation Notes

Hand-write headers.

Files or modules likely to be affected

contracts/lending/src/lib.rs.

Dependencies

#4 (test split).

Difficulty level

EASY.

Estimated effort

XS (~1 hour).