Face-Rig Slice F — GUI 'Add ARKit Blendshapes' button + end-to-end with face capture (#869)

[fernandotonon]

Part of epic #889. Depends on Slice E.

Goal

The GUI entry point, closing the loop with face capture (#869).

• 'Add ARKit Blendshapes' button in the Edit-Mode Vertex Morph Animation Inspector section (gated on a humanoid face-mesh selection with no/partial ARKit targets). Runs FaceRigger (worker thread if slow), progress + cancel, undoable.
• After it runs, the targets appear in the Vertex Morph list AND the Epic: AI — Performance capture: video/webcam → facial morph + skeletal body animation (live preview + record) #869 Performance Capture panel's Face channel lights up (matchedChannelCount jumps to ~52) on the same entity — verify that hand-off.
• Non-humanoid selection: button disabled with an explanatory tooltip.

Acceptance

• Button adds the 52 targets to the selected head; they show in the morph list.
• The Performance Capture Face channel then matches them (end-to-end GUI: rig face -> live/record face capture on a mesh that had none).
• Undoable; disabled + explained on unsuitable meshes; Sentry ui.action.

[fernandotonon]

Shipped in the consolidated FaceRig epic PR #903 (merged). All slice code, pure-data tests, docs (docs/FACE_RIG.md), and CLI qtmesh facerig / MCP add_arkit_blendshapes / GUI (Vertex Morph → Add ARKit Blendshapes) surfaces are on master.