Curved text: measurable path geometry abstraction

[rowland]

Summary

Add a reusable measurable-path abstraction for ellipse support and later path-following text.

Scope

• Add an internal measurable-path interface that can answer:
  • total length
  • point at distance
  • tangent at distance
  • normal at distance
• Provide a circle-backed implementation first.
• Add generic helpers for piecewise cubic paths.
• Reuse existing arc and cubic support in pdf and svg where practical, while keeping ownership in the PDF/text geometry layer.
• Define an approximation tolerance and deterministic sampling strategy so tests stay stable.

Acceptance Criteria

• Add unit tests for path length and tangent/normal sampling.
• Ensure cubic approximation results are deterministic across runs.
• Confirm the existing circle implementation can switch to this abstraction without behavior regression.