MeshTransform: Slice C — Variants GUI (Inspector section, streaming results) + MCP `generate_mesh_variants`

[fernandotonon]

Part of #845 · Design: docs/MESH_TRANSFORM_DESIGN.md §1.4 · Depends on Slice B (#847).

GUI + MCP surface for mesh variants.

• Inspector Object-mode section "AI: Variations" shown for a selected mesh entity: count spinner, strength slider (labeled zones: subtle / redesign / same-family), seed field, guidance toggle, Generate + progress + Cancel.
• Worker thread per the MeshGenController pattern (UI stays responsive; mesh construction marshalled to the main thread). Per-variant progress ("variant 3/10 · step 12/30").
• Streaming results: each finished variant inserts into the scene immediately as a sibling node (<name>_var01…), offset in a row, restored to source world scale — don't wait for the whole batch.
• MCP generate_mesh_variants {entity_name?, count, strength, seed?, guidance?, output_path?} (heavy, ONNX-gated schema).
• Undo: one named undo group per batch ("AI: 10 variants of Goblin").
• Sentry ai.assist.variants (GUI/MCP surfaces); gamification noteFeature cluster mapping in MCPServer::callTool.
• Tests headless-safe; QML section follows PropertiesPanel.qml conventions (no new dialog window).