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/GUI/Tab.cpp

@@ -3079,6 +3079,12 @@ void TabPrint::build()
         option.opt.full_width = true;
         optgroup->append_single_option_line(option, "others_settings_plugin_picker");
 
+        optgroup = page->new_optgroup(L("Slicing Pipeline Plugin"), L"param_gcode", 0);
+        optgroup->hide_labels();
+        option = optgroup->get_option("slicing_pipeline_plugin");
+        option.opt.full_width = true;
+        optgroup->append_single_option_line(option, "others_settings_plugin_picker");
+
         optgroup = page->new_optgroup(L("Notes"), "note", 0);
         option = optgroup->get_option("notes");
         option.opt.full_width = true;