Implement SVG opacity support

[rowland]

Problem

Our renderer currently warns with svg: <style>: opacity is not yet implemented; rendering as opaque. Assets that rely on opacity, fill-opacity, or stroke-opacity lose intended transparency.

What implementation would require

• Carry opacity values all the way through style parsing and style inheritance, including internal stylesheet classes and inline style attributes.
• Audit the renderer so per-node opacity is applied consistently to fill, stroke, and text.
• Expose PDF transparency support if needed, likely via ExtGState entries for fill/stroke alpha and a way to scope those settings safely.
• Decide how combined opacity, fill-opacity, and stroke-opacity multiply together.
• Add visual tests covering partially transparent shapes and text over other content.

Workarounds

• Export fully opaque SVG variants where acceptable.
• Flatten translucent artwork to PNG if the design depends on layered transparency.
• When only a few assets use opacity for subtle effects, replace those effects with solid-color approximations.