Fix shared plug calls under many parallel slots

Multi-slot stations that share one instrument plug across slots could see phases fail with "Connection reset by peer" when several slots called the plug at the same instant.

• Fixed the plug service dropping simultaneous calls: every call is now accepted immediately and executed one at a time, verified with 80 slots on a single shared power supply
• Improved plug shutdown and health checks so a long-running or abandoned instrument call no longer blocks them
• Added a safeguard that skips a queued call whose phase already timed out, so it never runs late on the instrument