At front startup _build_engines runs vulkaninfo to enumerate GPUs; on a
wedged Polaris card that process blocks UNINTERRUPTIBLY (D-state), and
subprocess.run(timeout=) can't kill a D-state child — so the front's
uvicorn startup hung forever ("Waiting for application startup"),
taking down the healthy NVIDIA engine too (502 everywhere).

Add _call_with_timeout: runs the probe in a daemon thread and STOPS
WAITING after a wall-clock timeout, leaving the un-killable call
orphaned instead of blocking. Wrap vulkan_devices() (12s) so startup
always completes, and _amd_stats() (6s) so a hung Radeon can't freeze
the health/thermal poll threads that also monitor the healthy card.
A wedged engine now degrades to "that engine down", not "everything down".
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014S8VtAvG499SsCbeESRK7V