Web-ready SVG export

Gauss now supports a web-ready export mode that strips editor metadata for publishing workflows.

• Metadata stripping: web-ready export removes xmlns:gauss, all gauss:* attributes, and preserved <metadata> payload content.
• Rendering fidelity: geometry (d), paint, opacity, and SVG resource references remain intact so that rendered output matches the authored artwork.
• Plain-SVG import behaviour: importing web-ready output restores shapes with default metadata (name = None, locked = false, hidden = false, and no opaque gauss:* attributes).

• Save SVG remains metadata-preserving for edit round-trips.
• Export web-ready SVG is the dedicated metadata-stripping command surface. The same behaviour is available programmatically via gauss::svg::export::export_svg_with_resources_web_ready() and gauss::svg::export::export_svg_with_resources_web_ready_checked().