feat: Apple Silicon (Metal) deployment guide

[itigges22]

Description

llama.cpp supports Metal for Apple Silicon acceleration. We need a deployment guide and potentially a Dockerfile for macOS users.

Requirements

• Test Qwen3.5-9B-Q6_K on Apple Silicon (M1/M2/M3/M4 with 16GB+ unified memory)
• Document bare-metal setup for macOS (Homebrew llama.cpp + Python services)
• Verify self-embeddings work via Metal backend
• Benchmark generation speed vs CUDA
• Add macOS section to docs/SETUP.md
• Consider Docker Desktop for Mac limitations (no GPU passthrough) — may need bare-metal-only guide

Context

Many developers run macOS. Metal support in llama.cpp is mature. The main question is whether Docker or bare-metal is the right approach (Docker Desktop doesn't support GPU passthrough on macOS).