fix(mlx): correct Apple Silicon GPU detection for issue #87

[Michael-A-Kuykendall]

Summary

• Fixes Apple Silicon hardware detection for users with M3 Pro and other Apple Silicon Macs
• Resolves issue Cannot get apple gpu info #87 where users received "MLX Backend: Not available" despite having supported hardware
• Separates hardware detection from Python package installation checks
• Enhances gpu-info output with actionable guidance

Problem

Users with Apple Silicon Macs (M3 Pro confirmed) were seeing:

🍎 MLX Backend: Not available (requires Apple Silicon)

This was misleading because:

1. They DO have Apple Silicon hardware
2. The check was incorrectly looking for MLX Python packages instead of hardware

Solution

Before (incorrect logic):

Hardware check: ✅ macOS + aarch64 
Python check: ❌ MLX packages not found
Result: "Not available (requires Apple Silicon)" ← WRONG

After (correct logic):

Hardware check: ✅ macOS + aarch64 
Result: "Hardware supported (Apple Silicon detected)"
Python check: ❌ MLX packages not found  
Guidance: "Install with: pip install mlx-lm"

Changes

• MLXEngine::check_mlx_availability() now only checks hardware (macOS + aarch64)
• Added MLXEngine::is_hardware_supported() for clear hardware detection
• Enhanced gpu-info output distinguishes hardware vs software availability
• Apple Silicon users get specific installation guidance
• Added comprehensive regression tests

Test Plan

• ✅ Added regression tests for Apple Silicon detection
• ✅ Tests verify hardware detection works independently of Python packages
• ✅ Tests ensure graceful handling of missing MLX Python packages
• ✅ Verified compilation on both default and MLX features

Expected User Experience

On Apple Silicon with MLX installed:

🍎 MLX Backend: ✅ Available (Apple Silicon + MLX installed)

On Apple Silicon without MLX (the #87 case):

🍎 MLX Backend: ⚠️  Hardware supported (Apple Silicon detected)
   📦 MLX Python packages not found  
   💡 Install with: pip install mlx-lm

On non-Apple Silicon:

🍎 MLX Backend: ❌ Not supported (requires Apple Silicon macOS)

-Mike