Draw rulers for all zero-corner states

[pascalpp]

Summary

Update horizontal and vertical ruler drawing so all four zero-corner states render correctly.

Implementation Notes

• Flip Horizontal reverses horizontal number growth.
• Flip Vertical reverses vertical number growth.
• Horizontal tick/label side depends on the vertical half of the zero corner.
• Vertical tick/label side depends on the horizontal half of the zero corner.
• Mouse tick labels should measure from the active zero edge.
• Do not add an explicit 0 label in this version.

Acceptance Criteria

• Ticks, numbers, unit labels, and mouse labels render correctly for top-left, top-right, bottom-left, and bottom-right.
• The current default drawing remains unchanged.
• Label placement avoids the resize handle and remains readable in each corner state.
• Focused layout/helper tests cover mirrored coordinates and mouse readout math.

[pascalpp]

Closed by #202