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.hpp

@@ -1571,6 +1571,7 @@ PRINT_CONFIG_CLASS_DERIVED_DEFINE(
     ((ConfigOptionString,             filename_format))
     ((ConfigOptionStrings,            post_process))
     ((ConfigOptionStrings,            post_process_plugin))
+    ((ConfigOptionStrings,            slicing_pipeline_plugin))
     ((ConfigOptionString,             printer_model))
     ((ConfigOptionFloat,              resolution))
     ((ConfigOptionFloats,             retraction_minimum_travel))