Three.js Compatibility Fallbacks

Treat fallback as an explicitly authorized product branch. Canonical WebGPU/TSL work and native WebGPU quality tuning remain with their owning skills.

1. Activate the branch

Record all of the following before selecting an implementation:

• the user's explicit request for unavailable-WebGPU fallback;
• proof from an initialized WebGPURenderer that renderer.backend.isWebGPUBackend !== true on the named target;
• the canonical owner and exact feature;
• the accepted scope of visual, physical, temporal, interaction, performance, and maintenance loss;
• the compatibility branch's maintenance owner.