Keep instruments connected between runs

Reconnecting to instruments between units costs time on every run — slow VISA sessions, TCP links, or serial handshakes were re-established for each unit tested on a station.

• Added scope: station for plugs: the connection opens once, is shared by every run on the station, and closes when the station stops
• Health-checked the held instance before each run, respawning it if the process died, the plug definition changed, or a new deployment was applied
• Renamed plug and phase scopes to slot / execution / station; the legacy each / all spellings still parse