Introduce zero-corner geometry helpers

[pascalpp]

Summary

Centralize the geometry for the shared ruler zero corner so drawing, resizing, alignment, and grouping all use the same definitions.

Implementation Notes

• Model four zero-corner states: top-left, top-right, bottom-left, bottom-right.
• Derive each ruler’s zero point, growth direction, tick/label side, resize-handle side, and frame placement from that model.
• Prefer pure helper functions/types that are easy to unit test.
• Avoid scattering left/right/top/bottom conditionals through controller and view code.

Acceptance Criteria

• A single helper layer can answer zero point, tick side, growth direction, resize side, and frame placement for both orientations.
• Tests cover all four zero-corner states.
• Existing default top-left behavior is represented without special casing outside the helper layer.

[pascalpp]

Closed by #207