Fix VOLTage:RANGe? timeout and EFF overflow blocking sweeps

- Wrap get_voltage_range() in try/except since VOLTage:RANGe? is not
  supported on IT6537D — skip range check gracefully if unavailable
- Exclude derived EFF* values from auto-range error threshold check
  in _wait_meter_ready(), as efficiency overflows at near-zero power

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

testbench/bench.py

@@ -183,11 +183,15 @@ class MPPTTestbench:
             result = self.meter.measure(*self.METER_ITEMS)
             values = result.values
 
-            # Check if any value is an error code
-            if all(abs(v) < ERROR_THRESHOLD for v in values.values()):
+            # Check if any ranged value is an error code.
+            # Derived values like EFF* are excluded – they overflow when
+            # input/output power is near zero, which is normal.
+            _DERIVED = {"EFF1", "EFF2", "EFF3", "EFF4"}
+            ranged = {k: v for k, v in values.items() if k not in _DERIVED}
+            if all(abs(v) < ERROR_THRESHOLD for v in ranged.values()):
                 return values
 
-            bad = [k for k, v in values.items() if abs(v) >= ERROR_THRESHOLD]
+            bad = [k for k, v in ranged.items() if abs(v) >= ERROR_THRESHOLD]
             print(f"    (auto-ranging: {', '.join(bad)} -- retrying {attempt + 1}/{max_retries})")
 
             # Keep supply alive during the wait so it doesn't
@@ -578,19 +582,25 @@ class MPPTTestbench:
 
         Raises ValueError if the configuration is invalid.
         """
-        supply_max_v = self.supply.get_voltage_range()
-        if max_voltage > supply_max_v:
-            raise ValueError(
-                f"Requested voltage {max_voltage:.1f}V exceeds supply "
-                f"range {supply_max_v:.1f}V"
-            )
+        # VOLTage:RANGe? is not supported on all models (e.g. IT6537D),
+        # so treat it as optional.
+        try:
+            supply_max_v = self.supply.get_voltage_range()
+            if max_voltage > supply_max_v:
+                raise ValueError(
+                    f"Requested voltage {max_voltage:.1f}V exceeds supply "
+                    f"range {supply_max_v:.1f}V"
+                )
+            range_str = f"(range {supply_max_v:.0f}V)"
+        except Exception:
+            range_str = "(range query N/A)"
 
         max_power = max_voltage * current_limit
         print(
             f"  Supply check: V_max={max_voltage:.1f}V, "
             f"I_limit={current_limit:.1f}A, "
             f"P_max={max_power:.0f}W "
-            f"(range {supply_max_v:.0f}V)"
+            f"{range_str}"
         )
 
         # CP mode: at min voltage the load draws max current