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/Config.cpp

@@ -1626,6 +1626,11 @@ void ConfigBase::save_plugin_collection(const std::string& opt_key, const Config
             append_ref(val, "post-processing");
     } else if (opt_key == "printer_agent") {
         append_ref((dynamic_cast<const ConfigOptionString *>(opt))->value, "printer-connection");
+    } else if (opt_key == "slicing_pipeline_plugin") {
+        if (const auto* vec = dynamic_cast<const ConfigOptionStrings*>(opt)) {
+            for (const std::string& val : vec->vserialize())
+                append_ref(val, "slicing-pipeline");
+        }
     }
     // Extend for other plugin-backed settings as needed.
 }