Emit clipped `<image href>` for fills with a registered media URL instead of Skia base64 embeds. Register URLs from the frontend and exporter via store_image_url. Cover rect, path, frame, and text (glyph clip) cases in SVG export tests. Closes #11383
Introduce a minimal vector SVG exporter via render-wasm: solid fills, native opacity/blend wrappers, frame clip-content, FFI/frontend wiring, insta snapshots, and a visual preview gallery. Defer save_layer workarounds (shadows, blur, masks, special strokes) to follow-up PRs.
