Add SVG parsing and rendering support

[Copilot]

Adds a pure-Rust SVG renderer (no new external dependencies) so .svg files can be drawn onto a canvas via a new draw_svg CLI command.

New module: canvas/src/svg.rs

• XML parser – zero-dependency tokenizer/parser; handles attributes, nested elements, XML entities, comments, CDATA
• SVG path flattener – full command set (M/L/H/V/C/S/Q/T/A/Z + relative variants); cubic/quadratic béziers use adaptive De Casteljau subdivision; arcs use SVG spec §B.2.4 endpoint-to-center parameterization
• Shape renderers – <rect> (+ rounded corners), <circle>, <ellipse>, <line>, <polyline>, <polygon>, <path>, <g>
• Style inheritance – fill, stroke, stroke-width, opacity, fill-opacity, stroke-opacity cascade through <g> groups; inline style="" + presentation attributes
• Transform engine – translate, scale, rotate, matrix, skewX/Y with full affine matrix concatenation
• viewBox – coordinate mapping with preserveAspectRatio="xMidYMid meet" (default)

Public API (canvas/src/lib.rs)

// Render SVG bytes → ImageData at given pixel size (0 = intrinsic)
pub fn render_svg(svg_bytes: &[u8], dest_width: u32, dest_height: u32) -> Option<ImageData>;

// Draw SVG directly onto an existing Context2D
pub fn draw_svg(ctx: &mut Context2D, svg_bytes: &[u8], dx: f64, dy: f64, dw: u32, dh: u32) -> bool;

CLI (cli/src/main.rs)

draw_svg <path> <dx> <dy>           # draw at intrinsic SVG size
draw_svg <path> <dx> <dy> <dw> <dh> # draw scaled to dw×dh pixels