Add memoized LTML canvas form rendering

[rowland]

Summary

• Add memoized canvas/form capture so repeated LTML draw content can be rendered once and reused as a form XObject.
• Thread memoization support through the LTML writer stack, PDF form handling, and canvas capture/draw paths.
• Update canvas drawing, link/index handling, and sample output to exercise the new memoized flow.

Testing

• Added and updated unit/integration coverage for form memoization, canvas drawing, and layout behavior.
• Verified the affected PDF and LTML test suites pass locally, including the new canvas memoization scenarios.