Curved text: circle text public API and first implementation

[rowland]

Summary

Ship the first user-facing curved-text feature: text on a circle.

Scope

• Add a circle-specific DocWriter/PageWriter API with:
  • center
  • radius
  • start angle
  • optional sweep or direction
  • text
  • horizontal anchor
  • vertical anchor
  • readable orientation mode
• Implement circle sampling analytically rather than through generic path approximation.
• Use current font metrics and vertical text alignment machinery as the baseline-offset source.
• Add at least one runnable sample under samples/.

Acceptance Criteria

• Add unit tests for top, bottom, left, and right arc cases.
• Add PDF-level tests asserting emitted transforms and text matrices are stable.
• Add a sample PDF that demonstrates common business-graphics use cases.