test(svg-renderer): add visual-fidelity test for sanitizeSvgText

Render the cat costume SVG twice — once raw, once after sanitizeSvgText —
and assert the canvases are pixel-identical. Also pin a committed PNG
snapshot of the sanitized render so future drift in sanitizer, fonts, or
canvas/librsvg shows up as a test failure rather than a silent
regression. Pulls in node-canvas as a devDep.

Author: cwillisf