Fix: Issue #2291 - Resolve registration difficulty latching

[Q-YZX0]

Description

Fixed the registration difficulty adjustment logic in block_step.rs to correctly handle cases where registrations are below target. Previously, the logic for Case C (POW > Burn) and Case D (Burn > POW) was swapped, causing difficulty to decrease when it should have increased (or vice versa), leading to a latching behavior.

Related Issue(s)

• Closes Subnet registration block 7105263 - registration price decreased instead of expected increase #2291

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Breaking Change

N/A

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run ./scripts/fix_rust.sh to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Screenshots (if applicable)

N/A

Additional Notes

Verified logic against the intended difficulty adjustment specifications.