PartOps Slice E — CLI/MCP parity for segmented part operations

[fernandotonon]

Goal

Expose segmented part operations outside the GUI.

Parent epic: #859

Scope

CLI:

• qtmesh segment model.fbx --write-labels labels.json
• qtmesh segment model.fbx --split-parts -o out.glb
• qtmesh segment model.fbx --explode-parts --explode-distance 0.15 -o out.glb
• qtmesh segment model.fbx --print-split --peg-radius 1.5 --peg-depth 4 -o out.stl

MCP:

• Extend segment_mesh with return_face_labels.
• Add split_mesh_by_segments.
• Add explode_mesh_parts.
• Add join_mesh_parts.
• Add prepare_print_split.

Acceptance Criteria

• GUI, CLI, and MCP produce equivalent segmented submesh output.
• Missing model/offline paths use existing segmentation fallback behavior.
• MCP returns structured part counts, created names, and warnings.

Tests

• CLI split-parts fixture.
• MCP structured result fixture.
• No-model fallback test.