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/libslic3r/PrintConfig.cpp

@@ -5122,6 +5122,16 @@ void PrintConfigDef::init_fff_params()
     def->mode = comAdvanced;
     def->set_default_value(new ConfigOptionStrings());
 
+    def = this->add("slicing_pipeline_plugin", coStrings);
+    def->label = L("Slicing Pipeline Plugin");
+    def->tooltip = L("Python plugin(s) invoked at each slicing pipeline step to read and modify intermediate slicing data. Research/experimental.");
+    def->gui_type = ConfigOptionDef::GUIType::plugin_picker;
+    def->plugin_type = "slicing-pipeline";
+    def->support_plugin = true;
+    def->full_width = true;
+    def->mode = comAdvanced;
+    def->set_default_value(new ConfigOptionStrings());
+
     def = this->add("printer_model", coString);
     def->label = L("Printer type");
     def->tooltip = L("Type of the printer.");