docs: streamline README — MetalRT M3+ warning, cleaner layout

- Add M3+ requirement notice for MetalRT with M1/M2 coming soon
- Position MetalRT as the flagship engine by RunAnywhere
- Remove redundant benchmark tables (was listed twice)
- Consolidate model listings into compact format
- Remove excessive badge clutter
- Cleaner feature sections without repetition

Made-with: Cursor

Author: shubhammalhotra28