PartOps Slice A — Segmentation preview and editable part groups

[fernandotonon]

Goal

Extend current "Select by Part (AI)" into a reusable segmentation preview workflow.

Parent epic: #859

Scope

• Store per-face segmentation labels for the active edit mesh.
• Show detected groups in Edit Mode: head, torso, left/right arms, left/right legs, unknown.
• Let users highlight, select, hide/show, rename display names, and exclude groups before splitting.
• Keep current one-click Select by Part behavior working.

Acceptance Criteria

• Segmentation can run once and be reused without rerunning the model.
• Selecting a group selects its faces.
• Excluded groups are ignored by downstream split/explode commands.
• Exiting Edit Mode clears stale segmentation state.
• Rig-prior, ONNX, and geometric fallback behavior remain unchanged.

Tests

• Unit test face-label grouping.
• Controller test that cached labels clear on edit-mode exit.
• Regression test current Select by Part behavior.