feat(plugin): add the slicing-pipeline plugin capability

Introduces a plugin capability that runs Python at the seams of Print::process(),
letting a plugin read and rewrite slicing state as it is computed.

- New slicing_pipeline_plugin config option; selected plugin refs are serialized
  into the print manifest.
- Print gains an injectable hook fired at each pipeline step (posSlice,
  posPerimeters, posInfill, ...). It is a no-op when unset, fires only on genuine
  (re)computation, and never on the use-cache path.
- orca.slicing submodule: SlicingPipelineCapabilityBase plus a trampoline and a
  Step enum. Capabilities read the live graph through zero-copy int64 numpy views
  (contour/holes geometry with unscaled coordinates, flattened toolpath data) and
  edit it through 2D-geometry mutators with cache-invariant refresh.
- GUI dispatcher runs capabilities during slicing under the GIL, turns plugin
  errors into slicing errors, honors cancellation, and adds the plugin picker.
- Ships the InsetEverySlice sample plugin and binding/hook tests.

src/slic3r/plugin/PythonPluginBridge.cpp

@@ -16,6 +16,7 @@
 #include "pluginTypes/gcode/GCodePluginCapability.hpp"
 #include "pluginTypes/printerAgent/PrinterAgentPluginCapability.hpp"
 #include "pluginTypes/script/ScriptPluginCapability.hpp"
+#include "pluginTypes/slicingPipeline/SlicingPipelinePluginCapability.hpp"
 
 namespace py = pybind11;
 
@@ -294,6 +295,7 @@ void bind_python_api(pybind11::module_& m)
                            .value("Exporter", PluginCapabilityType::Exporter)
                            .value("Visualization", PluginCapabilityType::Visualization)
                            .value("Script", PluginCapabilityType::Script)
+                           .value("SlicingPipeline", PluginCapabilityType::SlicingPipeline)
                            .value("Unknown", PluginCapabilityType::Unknown)
                            .export_values();
 
@@ -337,6 +339,7 @@ void bind_python_api(pybind11::module_& m)
     GCodePluginCapability::RegisterBindings(m, pluginTypes);
     PrinterAgentPluginCapability::RegisterBindings(m, pluginTypes);
     ScriptPluginCapability::RegisterBindings(m, pluginTypes);
+    SlicingPipelinePluginCapability::RegisterBindings(m, pluginTypes);
     PluginHostApi::RegisterBindings(m);
     BOOST_LOG_TRIVIAL(debug) << "Registered ScriptPluginCapability Python bindings";