Curved text: LTML curved-text syntax planning

[rowland]

Summary

Plan the LTML curved-text surface without implementing it yet.

Scope

• Decide whether curved text belongs on <label>, a new <textpath> tag, or a shape-owned child pattern.
• Map LTML-friendly attributes to the PDF API concepts:
  • circle vs ellipse vs future path
  • anchor/origin
  • readable orientation
  • start angle and direction
• Use SVG text-on-path only as reference vocabulary, not as a compatibility target.
• Document which pieces should wait until the PDF API has stabilized.

Acceptance Criteria

• Add a short syntax proposal in ltml/SYNTAX.md or an adjacent design note.
• Do not add parser or widget implementation in this issue.
• Clearly document what LTML will defer until after PDF API validation.