Implement dynamic jump-rate curve in lending pool

[nanaf6203-bit]

Problem Statement

Today: linear (utilisation / 50). Realistic lending uses piecewise jump rates.

Why it matters

Competitive with industry expectations.

Technical Context

Replace with kink at 80%, slope 2x above kink; configurable per pool.

Expected Outcome

Kink-based rate curve.

Acceptance Criteria

1. Property tests around kink.
2. SDK helpers.
3. Docs show formula.
4. Backwards compatible default.
5. Kani harness.

Implementation Notes

Math refactor.

Files or modules likely to be affected

contracts/lending/src/lib.rs.

Dependencies

None.

Difficulty level

HARD.

Estimated effort

L (~1 week).