refactor(plugin): split orca.host bindings into host/ by domain

PluginHostApi.cpp had grown into one TU holding the module entry point plus
three unrelated domains (presets, model/mesh graph, app access), and
PluginHostSlicing.cpp mixed ownable geometry value types with the
non-owning live print graph. Reorganize the orca.host surface into
plugin/host/ with one registrar per domain:

- PluginHost.hpp/.cpp        entry point (replaces PluginHostApi)
- PluginHostBindings.hpp     internal per-domain registrar declarations
- PluginHostGeometry.cpp     BoundingBox, Point, Polygon, ExPolygon + ndarray parsing
- PluginHostMesh.hpp/.cpp    TriangleMesh snapshot (own TU ahead of planned
                             mesh construct/mutate APIs)
- PluginHostPresets.cpp      Preset, PresetCollection, PresetBundle
- PluginHostModel.cpp        scene graph: Model, ModelObject, ModelInstance, ModelVolume
- PluginHostApp.cpp          Plater + plater()/model()/preset_bundle() accessors
- PluginHostSlicing.cpp      live print graph only, now with a single lifetime story
- PluginHostUi.hpp/.cpp      moved unchanged

PluginBindingUtils.hpp stays at plugin/ root: it is shared with pluginTypes/
and tests, not host/-specific.

No Python-visible change: same submodules, class names and docstrings.
Verified with slic3rutils and fff_print suites.

src/slic3r/plugin/PythonPluginBridge.cpp

@@ -10,7 +10,7 @@
 #include <pybind11/stl.h>
 
 #include "PythonInterpreter.hpp"
-#include "PluginHostApi.hpp"
+#include "host/PluginHost.hpp"
 #include "PyPluginPackage.hpp"
 #include "PyPluginTrampoline.hpp"
 #include "pluginTypes/printerAgent/PrinterAgentPluginCapability.hpp"
@@ -337,7 +337,7 @@ void bind_python_api(pybind11::module_& m)
     PrinterAgentPluginCapability::RegisterBindings(m, pluginTypes);
     ScriptPluginCapability::RegisterBindings(m, pluginTypes);
     SlicingPipelinePluginCapability::RegisterBindings(m, pluginTypes);
-    PluginHostApi::RegisterBindings(m);
+    PluginHost::RegisterBindings(m);
     BOOST_LOG_TRIVIAL(debug) << "Registered ScriptPluginCapability Python bindings";
 
     m.def(