Add plot-sweep command, degauss buttons, sweep ramp-down, and time estimate

- Add offline `plot-sweep` CLI command: generates efficiency overlay,
  heatmap, and power loss plots from sweep CSV (no instruments needed)
- Add degauss buttons to CH5/CH6 in GUI (sends :DEMAg SCPI command)
- Gradual load ramp-down at end of 2D sweep to avoid transients
- Live sweep time estimate in GUI based on grid size × settle time
- Update HIOKI submodule to include degauss CLI command

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

testbench/gui_workers.py

@@ -43,6 +43,7 @@ class Cmd(Enum):
     SET_CURRENT_RANGE = auto()
     SET_VOLTAGE_AUTO = auto()
     SET_CURRENT_AUTO = auto()
+    DEGAUSS = auto()
 
     # System
     SET_INTERVAL = auto()
@@ -205,6 +206,10 @@ class InstrumentWorker(threading.Thread):
                     bench.meter.set_voltage_auto(args[0], args[1])
                 case Cmd.SET_CURRENT_AUTO:
                     bench.meter.set_current_auto(args[0], args[1])
+                case Cmd.DEGAUSS:
+                    channels = args[0] if args else [5, 6]
+                    items = ",".join(f"I{ch}" for ch in channels)
+                    bench.meter.write(f":DEMAg {items}")
 
                 # System
                 case Cmd.SET_INTERVAL: