GPU detection refactor:
- Major rewrite of gpu_detector.go with unified detection interface
- Support for NVIDIA (NVML), AMD (ROCm), and Apple Metal
- Runtime GPU capability querying for scheduler matching
macOS improvements:
- gpu_macos.go: native Metal device enumeration and memory queries
- Support for Apple Silicon (M1/M2/M3) unified memory reporting
- Fallback to system profiler for Intel Macs
Testing infrastructure:
- Add gpu_detector_mock.go for testing without hardware
- Update gpu_golden_test.go with platform-specific expectations
- Cross-platform GPU info validation
