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/bench.py

@@ -739,10 +739,26 @@ class MPPTTestbench:
                 v += v_step
 
         finally:
+            # Ramp load down gradually to avoid sudden transients
+            cur_load = ll - l_step if n > 0 else l_start
+            ramp_steps = max(int(abs(cur_load - l_start) / abs(l_step)), 1) if l_step != 0 else 1
+            ramp_steps = min(ramp_steps, 10)  # cap at 10 steps
+            if abs(cur_load) > abs(l_start) and ramp_steps > 1:
+                decrement = (cur_load - l_start) / ramp_steps
+                print(f"\n  Ramping load down from {cur_load:.1f} to "
+                      f"{l_start:.1f}{unit} in {ramp_steps} steps...")
+                for i in range(1, ramp_steps + 1):
+                    step_val = cur_load - decrement * i
+                    self._apply_load_value(load_mode, step_val)
+                    time.sleep(0.5)
+            else:
+                self._apply_load_value(load_mode, l_start)
+                time.sleep(0.5)
+
             self.load.load_off()
             self.supply.set_voltage(IDLE_VOLTAGE)
             print(
-                f"\n  Load OFF. Supply returning to {IDLE_VOLTAGE:.0f}V "
+                f"  Load OFF. Supply returning to {IDLE_VOLTAGE:.0f}V "
                 f"(output stays ON)"
             )