feat: build CI docker images for linux/arm64 (Graviton)
Add arm64 native builds using the aws-m8g-l-cache runner group.
Each image (quality, consistency, custom-tokenizers, torch-light,
exotic-models, examples-torch) is now built on both amd64 and arm64
in parallel, then merged into a multi-arch manifest by a dedicated
merge job.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>