Object: do not include MRBox.h

[Fedr]

MRObject.h no longer includes MRBox.h. The only thing in the header that needed Box3f complete was the inline getWorldBox() body, so that moves to the .cpp (MRMESH_API virtual, as the override declarations in the holders already are); Box3f in the two remaining declarations is satisfied by MRMeshFwd.h.

MRVisualObject.h now includes MRBox.h itself - it has an inline computeBoundingBox_() returning Box3f() and a std::optional<Box3f> member - which keeps every visual-object descendant supplied. MRObject.cpp includes it for getWorldTreeBox().

Verified with a cl /Zs sweep of MRMesh, MRViewer, MRVoxels and MRSymbolMesh: failing-TU sets identical to the same sweep on master (4 known no-PCH artifacts in MRMesh, 82 in MRViewer, 0 elsewhere). An include-graph pass over the repo - traversing thirdparty and vcpkg headers this time, so std headers reached through MRBox.h (<algorithm>, <cassert>, <iosfwd>, <limits>, <type_traits>) are accounted for - leaves only MRObjectTransformWidget.h, whose const Box3f& parameters need no definition.

Note this overlaps #6274, which drops MRBox.h along with MRAffineXf3.h, MRViewportProperty.h and <array> and makes the same getWorldBox() and MRVisualObject.h changes; whichever lands second will need a trivial conflict resolution.