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/host/PluginHost.cpp

@@ -0,0 +1,26 @@
+#include "PluginHost.hpp"
+#include "PluginHostBindings.hpp"
+#include "PluginHostUi.hpp"
+
+namespace Slic3r {
+
+void PluginHost::RegisterBindings(pybind11::module_& module)
+{
+    auto host = module.def_submodule("host", "Host application API");
+
+    // Value types first so the docstring signatures of later registrars
+    // resolve to the bound Python names.
+    host_bindings::register_geometry(host);
+    host_bindings::register_mesh(host);
+    host_bindings::register_presets(host);
+    host_bindings::register_model(host);
+    host_bindings::register_app(host);
+
+    // UI: native dialogs and interactive HTML windows for plugins.
+    PluginHostUi::RegisterBindings(host);
+
+    // Slicing print-graph data model (Print, Layer, Surface, ...).
+    host_bindings::register_slicing(host);
+}
+
+} // namespace Slic3r