Implement snapping to gridlines of clip gain control points

### Description

When adding clip gain control points, if snapping is enabled, they should snap to gridlines according to the currently selected interval (bar, 1/2, etc.)

Start by locating the clip gain control-point handling and the existing snapping logic, then trace how the selected grid interval is applied. Add or update coverage for creating control points with snapping enabled at intervals such as bar and 1/2; done means points align with the currently selected gridlines.

Written by the indexing model from the issue text.