Mac in the headset: USB route keeps a configured host-key alias and falls back to the network

Review round 12: a failed USB-C tunnel now retries the normal path; an
existing HostKeyAlias wins; --host with --usb is rejected; the Steam
desktop-streaming claim is now 'untested' (Valve documents the desktop
showing when a game loses focus); the Show/cleanup overlap test blocks
for real (it fails without the lock). Verified live: with the cable out,
the route is the normal path.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

scripts/macview-bench.py

@@ -808,6 +808,8 @@ def main():
     args = p.parse_args()
     # The agent keeps the last 4096 frames (Stats.swift) and results are read
     # at the end: at 60 fps that's 68 s, less warm-up and the reporting wait.
+    if args.cmd != "compare" and args.host and args.usb:
+        p.error("--host and --usb pick the path two ways; use one")
     if args.cmd != "compare" and args.warmup + args.duration > 60:
         p.error("--warmup plus --duration can be at most 60 s (the agent keeps 4096 frames)")