Make ruler resizing direction-aware

[pascalpp]

Summary

Make resize handles and resize frame math follow the current zero-corner geometry.

Implementation Notes

• Resize handles live at the far end of each ruler’s measurement direction.
• Resizing keeps the zero edge fixed and grows/shrinks away from zero.
• Horizontal default behavior should still resize from the right with the left edge fixed.
• Vertical default behavior should still resize from the bottom with the top edge fixed.
• Flipped states should mirror that behavior around the selected zero edge.

Acceptance Criteria

• Resize handle position is correct for all four zero-corner states.
• Resize math preserves the zero edge for both orientations.
• Min/max size clamping still works.
• Existing resize tests continue to pass, with new tests for flipped horizontal and vertical behavior.

[pascalpp]

Closed by #203