Adds a Grok image backend (grok_generator.py) behind a generator factory, a
starter-set orchestrator (starter.py, orchestrate_starter.py, starter_manifest.json)
and a Grok PoC harness with proof renders. Updates the ranker/processor/composition
prompts and sprite-config; refreshes the sprite-gallery design preview.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>