Add Native Variant Authoring Support with UI and Python Helpers in OpenDCC(Loki)

[Ambassy]

Feature Request: Native Variant Authoring Support in Loki

Context: Currently, Loki lacks a native UI for authoring USD variant sets and variant-scoped opinions, forcing users to either manually edit USDA text files or write complex Python scripts using the low-level Sdf API. While the USD specification provides robust variant functionality for managing multiple configurations of assets (LODs, material variations, etc.), the Sdf.VariantSpec and related APIs have challenging constructors and undocumented patterns that make programmatic variant creation error-prone. Even experienced USD developers struggle with properly scoping prims and relationships within variant contexts, as the Python bindings don't expose intuitive methods like GetVariantEditContext() or allow straightforward prim creation within variant specs. This creates a significant barrier for technical artists and pipeline developers who need to build production-ready assets with combinatorial variants (e.g., mixing geometry LODs with material variations).

Requested Features: We propose adding a dedicated Variant Authoring Panel to Loki with the following capabilities: (1) A visual UI to create and manage variant sets on selected prims, with dropdowns to add/remove individual variants; (2) A "variant isolation mode" that allows users to select a specific variant, make edits (create prims, set attributes, bind materials), and have those opinions automatically scoped to that variant; (3) A variant preview system showing which opinions exist in each variant, with visual indicators for differences between variants; (4) Helper functions in the Python API (opendcc.cmds) such as cmds.create_variant_set(), cmds.add_variant(), and a context manager like with cmds.edit_variant(prim, "variantSet", "variantName"): that handles the complex Sdf layer manipulation internally. This would enable artists to build industry-standard USD assets with proper variant structures directly in Loki, eliminating the need for external tools or manual USDA editing, and significantly improving Loki's competitiveness as a USD authoring platform for production pipelines.