Add leverage math property tests

[Sikkra]

Resolves #50.

Summary:

• Added proptest coverage for leverage math with 1,000 cases per property.
• Covered step borrow bounds, final-step no-borrow behavior, total supply/borrow equity preservation, leverage upper bound, health-factor monotonicity in collateral factor, and no-panic behavior on extreme inputs.
• Changed loop_step_count to use saturating_add(1) so extreme u32 loop counts cannot overflow before the 21-step cap.

Verification:

• $env:CARGO_TARGET_DIR = Join-Path $env:TEMP "codex-turbolong-blend-proptests"; cargo test --manifest-path contracts\strategies\blend_leverage\Cargo.toml -j 1 leverage_property_tests -- --nocapture
• git diff --check

Note: using a target directory under %TEMP% avoids a Windows build-script failure seen when Cargo target artifacts are under the workspace path with spaces.