Split CanvasDocument into separate package

[Copilot]

Extracts Canvas rendering backend into dedicated canvas/ package, following established modularization pattern from geometry and color packages.

Changes

• New canvas/ package: CanvasDocument type, rendering methods, and 11 tests
• Type re-export: pub type CanvasDocument = @canvas.CanvasDocument in main package for backward compatibility
• Simplified wrapper: Main canvas.mbt reduced to new_canvas function forwarding to @canvas.new_canvas
• Package dependencies: Canvas package imports geometry and color packages
• Documentation: Added REFACTORING_CANVAS_PACKAGE.md

API Compatibility

Fully backward compatible. Existing code requires no changes:

// Works identically before and after refactoring
let doc = @vg.new_canvas(300.0, 200.0)
  .render_circle(@vg.Point::new(100.0, 100.0), 50.0, @color.red())
  .to_html("Demo")

Canvas package can also be imported directly if needed:

// Optional: direct import
let doc = @canvas.new_canvas(300.0, 200.0)

Testing

186 tests pass (175 existing + 11 new canvas package tests).

Original prompt

split CanvasDocument into a separate package

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