PartOps Slice B — Split segmented groups into real submeshes

[fernandotonon]

Goal

Convert accepted segmentation groups into actual Ogre submeshes.

Parent epic: #859

Scope

• Add shared helper SubMeshOps::splitByFaceGroups.
• Create one submesh per accepted part group.
• Preserve material by default; optionally assign generated part materials.
• Duplicate boundary vertices so split submeshes are independent.
• Preserve normals, UVs, vertex colors, tangents, bone weights, n-gon face cache, and UV seam bindings where applicable.
• Add a mesh-replacement undo command because current EditMeshTopologyCommand requires unchanged submesh count.

Acceptance Criteria

• A single-submesh humanoid can become Body.head, Body.torso, Body.left_arm, etc.
• Undo restores the original submesh layout.
• FBX/glTF export preserves new submesh boundaries.
• Skinned fixtures retain valid bone assignments.

Tests

• Split fixture by synthetic face labels.
• Attribute preservation test.
• Boundary vertex duplication test.
• Skinned mesh bone assignment test.