Fix CSG subtract for composite base meshes

[tracygardner]

Motivation

• Composite GLB bases were being reduced to a single child with material for CSG, which broke subtraction on multi-part models.
• The result mesh should keep material/visual reference from a material-bearing descendant while using the full composite geometry for CSG operations.

Description

• Introduced referenceMesh (the first descendant with a material or the base) and kept actualBase for geometry preparation in subtractMeshes, subtractMeshesMerge, and subtractMeshesIndividual.
• Continued to call prepareMeshForCSG(actualBase) so composite children are merged into a geometry-ready mesh while preserving the separate referenceMesh for copying materials and transforms.
• Replaced previous uses of actualBase in applyResultMeshProperties with referenceMesh so the result inherits the correct material and visual properties.
• Kept existing CSG cloning, merging, and cleanup logic unchanged to preserve prior behavior for subtraction approaches.

Testing

• No automated tests were run for this change.
• Manual validation was not recorded in automated test outputs.

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Codex Task