Split SVGDocument into separate svg package

[Copilot]

Extracts SvgDocument and all SVG rendering methods into a dedicated svg package following the established pattern used for geometry and color packages. Image::render_image_to_svg remains in the main vg package.

Changes

• New svg package: Created svg/types.mbt (SvgDocument struct), svg/svg.mbt (all rendering methods and helpers), and svg/moon.pkg.json (imports geometry and color)
• vg package updates: Added SvgDocument type re-export to types.mbt, reduced svg.mbt to only Image::render_image_to_svg, added svg package import
• Namespace migration: All SVG document creation and rendering now uses @svg namespace instead of @vg
• Documentation: Added REFACTORING_SVG_PACKAGE.md following existing refactoring docs pattern

Migration

// Before
let doc = @vg.new_svg(200.0, 200.0)
  .render_circle(Point::new(50.0, 50.0), 25.0, @color.red())

// After
let doc = @svg.new_svg(200.0, 200.0)
  .render_circle(Point::new(50.0, 50.0), 25.0, @color.red())

// Image method unchanged in vg package
let svg_string = img.render_image_to_svg(100.0, 100.0, 20)

Type references continue to work via re-export: SvgDocument or @vg.SvgDocument.

Original prompt

split SVGDocument into a separate package, note Image::render_to_svg should be kept in vg package

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