PartOps Slice C — Explode segmented parts into separate scene nodes and join back

[fernandotonon]

Goal

Add Blender-style explode/join workflow for segmented or submesh-based parts.

Parent epic: #859

Scope

• Add Explode Parts from either existing submeshes or segmentation groups.
• Create one child scene node/entity per part.
• Preserve world transform, material, and vertex attributes.
• Add configurable explode offset for visual inspection.
• Add Join Parts to merge selected exploded entities into one mesh, baking transforms into vertices.
• Reuse/extend SceneOps: Slice C — Join (merge) selected submeshes into one #626 join semantics for vertex declaration and material conflict handling.

Acceptance Criteria

• Exploded parts are independent selectable scene nodes.
• Moving exploded parts and joining back bakes transforms correctly.
• Undo restores pre-explode scene state.
• Joined mesh exports as one entity with expected submeshes/materials.

Tests

• Explode a multi-part fixture.
• Move one part, join, assert baked vertex positions.
• Undo/redo scene state test.