Fix VISA timeout in 2D sweep: join worker thread before starting sweep

The worker thread was signaled to stop but never joined, causing bus
collisions when the sweep thread started talking to instruments while
the worker was still mid-query. Now:

- worker.join(timeout=10) before sweep starts
- 0.5s delay + clear supply error queue before first command
- join sweep thread before restarting worker after sweep completes
- Also join worker on disconnect

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

testbench/gui.py

@@ -724,6 +724,7 @@ class TestbenchGUI(tk.Tk):
         self._stop_log()
         if self.worker:
             self.worker.stop()
+            self.worker.join(timeout=5)
             self.worker = None
         if self.bench:
             try:
@@ -1130,9 +1131,11 @@ class TestbenchGUI(tk.Tk):
             "success",
         )
 
-        # Stop the normal worker so the sweep owns the instruments
+        # Stop the normal worker and wait for it to finish so the
+        # sweep thread has exclusive access to the instrument bus
         if self.worker:
             self.worker.stop()
+            self.worker.join(timeout=10)
             self.worker = None
 
         # Restart _poll so it drains the sweep data queue
@@ -1190,6 +1193,13 @@ class TestbenchGUI(tk.Tk):
         elif l_start > l_stop and l_step > 0:
             l_step = -l_step
 
+        # Clear any stale state
+        time.sleep(0.5)
+        try:
+            bench.supply.get_error()
+        except Exception:
+            pass
+
         # Sanity check
         max_v = max(abs(v_start), abs(v_stop))
         min_v = min(abs(v_start), abs(v_stop))
@@ -1312,6 +1322,9 @@ class TestbenchGUI(tk.Tk):
     def _sweep_vi_done(self) -> None:
         self._btn_svi_run.config(state=tk.NORMAL)
         self._btn_svi_stop.config(state=tk.DISABLED)
+        # Wait for sweep thread to fully exit
+        if self._svi_thread:
+            self._svi_thread.join(timeout=5)
         self._svi_thread = None
         self._svi_stop_event = None
         # Restart normal worker polling