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.

tests/slic3rutils/test_plugin_host_api.cpp

@@ -5,6 +5,8 @@
 #include <libslic3r/TriangleMesh.hpp>
 #include <slic3r/plugin/PythonPluginBridge.hpp>
 
+#include "python_test_support.hpp"
+
 #include <pybind11/embed.h>
 #include <pybind11/pybind11.h>
 
@@ -14,30 +16,8 @@ namespace py = pybind11;
 
 namespace {
 
-void ensure_python_initialized()
-{
-    // Deliberately a bare scoped_interpreter rather than Slic3r::PythonInterpreter:
-    // `orca` is a PYBIND11_EMBEDDED_MODULE compiled into this test binary, so importing
-    // it needs no bundled stdlib/sys.path, and the deterministic assertions are
-    // independent of the host's Python. PythonInterpreter::initialize() expects the
-    // bundled Python home laid out next to the app bundle (lib/python3.12/encodings),
-    // which is not deployed beside the test binary, so using it here would fail to find
-    // a home on macOS/Linux. The optional numpy-backed assertions are guarded at runtime.
-    if (!Py_IsInitialized()) {
-        static py::scoped_interpreter interpreter;
-        (void) interpreter;
-    }
-}
-
-py::module_ import_orca_module()
-{
-    ensure_python_initialized();
-
-    // Force PythonPluginBridge.cpp into the test binary so the embedded
-    // PYBIND11_EMBEDDED_MODULE(orca, ...) registration is available.
-    (void) Slic3r::PythonPluginBridge::instance();
-    return py::module_::import("orca");
-}
+// import_orca_module() lives in python_test_support.hpp (shared with
+// test_slicing_pipeline_bindings.cpp).
 
 bool has_attr(const py::handle& object, const char* name)
 {