Handle grouped and ungrouped zero-corner flips

[pascalpp]

Summary

Implement the movement rules for Flip Horizontal and Flip Vertical when rulers are grouped or ungrouped.

Behavior

• Ungrouped: flipping redraws both rulers and updates tick/label sides, but does not move the non-flipped ruler.
• Grouped: flipping moves the non-flipped ruler so its zero point keeps the same zero-point offset relationship it had before the flip.
• Flipping one axis can affect the other ruler’s tick/label side.

Implementation Notes

• Capture zero-point relationship before changing the flip state.
• Apply the flip.
• If grouped, reposition the non-flipped ruler using the shared geometry helper.
• Keep child-window grouping behavior intact.

Acceptance Criteria

• Grouped and ungrouped behavior matches the rules above.
• Tests cover horizontal and vertical flips from each corner state where practical.
• Hidden ruler behavior is not broken.
• No unexpected movement happens when rulers are ungrouped.

[pascalpp]

Closed by #204