Curved text: internal glyph-on-path placement model

[rowland]

Summary

Build the internal glyph-on-path placement model that later curved-text APIs will use.

Scope

• Introduce an internal placement type that records per glyph or shaped cluster:
  • glyph ID or text slice
  • path distance
  • point (x, y)
  • tangent angle
  • baseline offset along the normal
• Keep the model internal to pdf for now.
• Base placement on actual glyph advance widths so the TTF/composite-font path stays correct.
• Reuse existing text matrix support instead of inventing a new PDF text primitive.
• Support emission as one rotated placement per glyph or shaped cluster as needed.

Acceptance Criteria

• Add unit tests for placement math with simple synthetic inputs.
• Add tests covering left/center/right anchor distribution along the path.
• Keep this issue internal-only with no public API yet.