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/slic3r/plugin/PluginDescriptor.hpp

@@ -4,6 +4,7 @@
 
 #include <algorithm>
 #include <cctype>
+#include <map>
 #include <optional>
 #include <string>
 #include <utility>
@@ -61,6 +62,7 @@ struct PluginDescriptor
     std::string entry_path;                             // Full path to the installed plugin entry file
     std::string entry_package;                          // Import package/module used for package-based loading
     std::vector<std::string> dependencies;              // Python dependency requirements declared by plugin package metadata
+    std::map<std::string, std::string> settings;        // [tool.orcaslicer.plugin.settings] table -> per-plugin params (ctx.params)
     std::vector<PluginChangelog> changelog;             // Cloud release changelog, sorted newest-first when available.
 
     std::string error;                     // Blocking error message. Non-empty means the plugin is in an error state.