feat(plugin): expose the slicing print-graph as raw orca.host classes + Twistify sample

Adds PluginHostSlicing, which registers the print-graph data model (Print,
PrintObject, Layer, LayerRegion, Surface, ExPolygon, extrusions, ...) into the
orca.host submodule in the same raw-class style as PluginHostApi's Model/Preset
graph, with shared helpers in PluginBindingUtils. SlicingPipelinePluginCapability
is trimmed to the capability surface (the standalone SlicingNumpy helper is folded
away). Adds the Twistify example plugin next to Inset and broadens the binding,
hook, and plugin-install tests.

src/libslic3r/Print.hpp

@@ -883,7 +883,7 @@ enum FilamentCompatibilityType {
 };
 
 enum class SlicingPipelineStep {
-    Slice, Perimeters, EstimateCurledExtrusions, Infill, Ironing, Contouring,
+    Slice, Perimeters, EstimateCurledExtrusions, PrepareInfill, Infill, Ironing, Contouring,
     SupportMaterial, DetectOverhangsForLift, SimplifyPath, WipeTower, SkirtBrim
 };