feat: Add native SVG to PNG/JPEG export using standard libraries

Implements SVG rasterization without external dependencies (oksvg/rasterx).
Uses golang.org/x/image/vector for antialiased rendering.

Features:
- Export to SVG (passthrough), PNG, and JPEG formats
- Configurable width, height, quality, and DPI
- Basic shape support: rectangles, circles, lines
- Color parsing for hex and named colors
- Antialiased circle rendering using 32-segment approximation

Implementation:
- parseSVG() uses encoding/xml for SVG parsing
- rasterize() uses x/image/vector.Rasterizer
- Standard image/png and image/jpeg encoders
- Helper functions for MIME types and file extensions

Limitations:
- Text rendering not yet implemented (requires font support)
- Path elements not yet implemented
- No transform, gradient, or advanced feature support

Added comprehensive tests validating all export formats and utilities.

Add StrokeDashArray support

Update README with PathBuilder and Markers documentation

- Add comprehensive PathBuilder usage examples
- Document all marker types and usage patterns
- Add testing and performance sections
- Include code coverage information
- Add performance benchmarks table