Curved text: ellipse text support

[rowland]

Summary

Extend curved text support from circles to ellipses.

Scope

• Add an ellipse-specific public API parallel to circle text.
• Define ellipse parameters in the same style as existing shape APIs: center + rx + ry + start angle + direction.
• Reuse the same horizontal anchor, vertical anchor, and readable-orientation options as circle text.
• Address spacing and orientation behavior on varying curvature without introducing glyph deformation.

Acceptance Criteria

• Add tests for major-axis and minor-axis starts, clockwise/counterclockwise traversal, and centered text.
• Add a sample output demonstrating top and bottom ellipse labels.
• Avoid breaking changes to the circle API.