feat: Added B&W/grayscale/color conversion modes, invert toggle, 10+ mode-aware sliders, mask regions, turnpolicy, and white preview background
Engine:
- preprocess() accepts conversion_mode (bw/grayscale/color), invert, mask_regions
- B&W: full pipeline → binary; Grayscale: skip threshold → 8-bit; Color: skip grayscale → BGR
- routes.py forces VTracer for non-binary modes, sets colormode appropriately
- potrace_trace() accepts turnpolicy param mapped to potrace constants
- 27 new tests in test_modes.py (modes, invert, masks, params, vectorization)
App:
- Mode selector tabs (B&W | Grayscale | Color) in ImportConvert
- Invert toggle (B&W only)
- ParameterSliders rewritten: grouped sections, 10+ mode-aware controls
- Debounce reduced from 300ms to 100ms
- Preview background changed to white
- Preset JSONs updated with turnpolicy, color_precision, layer_difference defaults
Tests: 126 app + 234 engine = 360 total, all pass. Zero TypeScript errors.
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