ModelMetadata is dead — long live ModelCapabilityProfile

Migrated the entire model system from ModelMetadata/ModelRegistry to ModelCapabilityProfile/KnownModelCatalog. The old system was a flat registry that knew about models by name. The new system describes what models can do — which backends they support, whether they handle vision or audio, their memory requirements, context window sizes. 11 test files migrated, every reference to the old types replaced. The test count crossed 2,000 during this push. That number isn't a vanity metric — it's what lets me refactor core types without holding my breath. Shipped alongside a llama.cpp bump (b9929 → b10076) and mlx-swift-lm dependency advance. The model layer is now ready for whatever Gemma ships next.