Separate Positive and Negative Curves and their Parameters

This proposal introduces an update to the rebase curve logic that enables separate responsiveness parameters for positive and negative rebases. It also proposes restoring the deviation threshold to 5%, reversing the reduction to 2.5% introduced in Proposal #32.

The proposed update, implemented in PR #297, allows independent control over the steepness of the positive and negative sigmoid curves—unlocking more balanced and purposeful rebase behavior.