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.py

@@ -410,6 +410,8 @@ class TestbenchGUI(tk.Tk):
         self._ch5_i_range.pack(side=tk.LEFT, padx=2)
         ttk.Button(rng5, text="Set", width=3,
                    command=lambda: self._set_range(5, "I", self._ch5_i_range)).pack(side=tk.LEFT)
+        ttk.Button(rng5, text="Degauss", width=7,
+                   command=lambda: self._send(Cmd.DEGAUSS, [5])).pack(side=tk.LEFT, padx=(8, 0))
         self._ch5_range_label = ttk.Label(input_frame, text="V: --- / I: ---", font=("Consolas", 9))
         self._ch5_range_label.pack(anchor=tk.W)
 
@@ -437,6 +439,8 @@ class TestbenchGUI(tk.Tk):
         self._ch6_i_range.pack(side=tk.LEFT, padx=2)
         ttk.Button(rng6, text="Set", width=3,
                    command=lambda: self._set_range(6, "I", self._ch6_i_range)).pack(side=tk.LEFT)
+        ttk.Button(rng6, text="Degauss", width=7,
+                   command=lambda: self._send(Cmd.DEGAUSS, [6])).pack(side=tk.LEFT, padx=(8, 0))
         self._ch6_range_label = ttk.Label(output_frame, text="V: --- / I: ---", font=("Consolas", 9))
         self._ch6_range_label.pack(anchor=tk.W)
 
@@ -570,6 +574,15 @@ class TestbenchGUI(tk.Tk):
 
         self._svi_status = ttk.Label(frame, text="Idle", font=("Consolas", 9))
         self._svi_status.pack(anchor=tk.W, pady=2)
+        self._svi_estimate = ttk.Label(frame, text="", font=("Consolas", 9))
+        self._svi_estimate.pack(anchor=tk.W)
+
+        # Bind entries to recalculate time estimate on change
+        for entry in (self._svi_v_start, self._svi_v_stop, self._svi_v_step,
+                      self._svi_l_start, self._svi_l_stop, self._svi_l_step,
+                      self._svi_settle):
+            entry.bind("<KeyRelease>", lambda _e: self._update_svi_estimate())
+        self._update_svi_estimate()
 
         self._svi_thread = None
         self._svi_stop_event = None
@@ -907,6 +920,37 @@ class TestbenchGUI(tk.Tk):
             self._svi_l_label.config(text="I start:")
         else:
             self._svi_l_label.config(text="P start:")
+        self._update_svi_estimate()
+
+    def _update_svi_estimate(self) -> None:
+        """Recalculate and display estimated sweep duration."""
+        try:
+            v_start = float(self._svi_v_start.get())
+            v_stop = float(self._svi_v_stop.get())
+            v_step = abs(float(self._svi_v_step.get()))
+            l_start = float(self._svi_l_start.get())
+            l_stop = float(self._svi_l_stop.get())
+            l_step = abs(float(self._svi_l_step.get()))
+            settle = float(self._svi_settle.get())
+            if v_step <= 0 or l_step <= 0:
+                raise ValueError
+            v_count = int(abs(v_stop - v_start) / v_step) + 1
+            l_count = int(abs(l_stop - l_start) / l_step) + 1
+            total = v_count * l_count
+            secs = total * settle
+            mins, s = divmod(int(secs), 60)
+            hrs, m = divmod(mins, 60)
+            if hrs:
+                time_str = f"{hrs}h {m:02d}m {s:02d}s"
+            elif m:
+                time_str = f"{m}m {s:02d}s"
+            else:
+                time_str = f"{s}s"
+            self._svi_estimate.config(
+                text=f"{total} points, ~{time_str} (settle only)"
+            )
+        except (ValueError, ZeroDivisionError):
+            self._svi_estimate.config(text="")
 
     def _set_range(self, channel: int, vi: str, combo: ttk.Combobox) -> None:
         """Set voltage or current range for a HIOKI channel."""
@@ -1286,6 +1330,22 @@ class TestbenchGUI(tk.Tk):
                     ll += l_step
                 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"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
+                    bench._apply_load_value(load_mode, step_val)
+                    time.sleep(0.5)
+            else:
+                bench._apply_load_value(load_mode, l_start)
+                time.sleep(0.5)
+
             bench.load.load_off()
             bench.supply.set_voltage(IDLE_VOLTAGE)