SVG Resource Definitions

Gauss now preserves shared SVG resource definitions in <defs> when opening and saving files.

• Supported resources: linear gradients, radial gradients, patterns, and symbols.
• Paint references: paths that use stroke="url(#...)" or fill="url(#...)" are loaded and saved with those references intact.
• Resource attribute fidelity: pattern and symbol attributes (for example, patternUnits, patternTransform, and preserveAspectRatio) are preserved when opening and saving.
• Paint-server opacity fidelity: stroke-opacity and fill-opacity are preserved for url(#...) paints.
• Validation: if an SVG path references a missing resource ID, Gauss reports an open error and keeps the currently loaded document unchanged.
• Save invariants: if the current document contains dangling gradient or pattern references, save fails with an explicit error instead of silently exporting none.

In this phase, Gauss supports resource round-tripping and rendering, but does not yet provide dedicated editing controls for gradients, patterns, or symbols.