Fix native macOS tailnet launch and Metal GPU probe (#756)

* macOS/Apple Silicon: detect Metal backend, surface MLX models, brew tmux hint

- hardware.py: add _detect_macos() via sysctl/system_profiler; report
  backend=metal + unified_memory on Apple Silicon instead of cpu_arm
- fit.py: add Apple Silicon (M1-M5) unified-memory bandwidths + metal
  FALLBACK_K so throughput estimates use the real bandwidth formula
- setup.py: Mac-specific 'brew install tmux' hint

Verified on M5 Pro 48GB: backend=metal, 273GB/s matched, 6 MLX models now
visible (were hidden), cuda still hides MLX, no new test failures.

* Fix native macOS tailnet launch and Metal GPU probe

---------

Co-authored-by: Elijah (Hermes) <hermes@local>

services/hwfit/fit.py

@@ -26,7 +26,8 @@ GPU_BANDWIDTH = {
     "m1 ultra": 800, "m1 max": 400, "m1 pro": 200, "m1": 68,
     "m2 ultra": 800, "m2 max": 400, "m2 pro": 200, "m2": 100,
     "m3 ultra": 800, "m3 max": 300, "m3 pro": 150, "m3": 100,
-    "m4 max": 410, "m4 pro": 273, "m4": 120,
+    "m4 max": 546, "m4 pro": 273, "m4": 120,
+    "m5 max": 546, "m5 pro": 273, "m5": 150,
 }
 
 # Pre-sort keys by length descending for correct substring matching