Add ZXing.Net decode round-trip scan-validation harness

The original plan (Phase 7) called for a ZXing.Net-based decode round-trip validation harness (dev/test only) to re-validate scannability for style combinations - especially important once smaller-fill-area shapes (pentagon, hexagon, star, etc.) are used at data-module scale, since sharp points can reduce effective fill area vs. circle/square.

This was explicitly deferred because it requires rasterizing the generated SVG to a bitmap (e.g. via SkiaSharp) before ZXing.Net can decode it - a bigger lift than the rest of the rendering work.

**Scope:**
- Add a rasterization step (SkiaSharp or similar) to convert `QrSvgRenderer` output to a bitmap in the test project.
- Add a ZXing.Net-based decode test harness that round-trips: encode -> render (various shape/color/logo combos) -> rasterize -> decode -> assert the decoded value matches the original input.
- Use this harness to validate scannability thresholds for new shapes as they're added (see related shape issues).