fix(dashboard): improve server singleton robustness

- Add reuse_address=True to TCP server start to allow quick rebind
  after process restart (TIME_WAIT state)
- Add _is_server_responsive() check to verify server is actually
  responding, not just trusting the is_running flag which can be stale
  if the server thread died unexpectedly

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/py_dvt_ate/app/dashboard/app.py

@@ -107,6 +107,16 @@ class TestProgress:
         return time.time() - self.started_at
 
 
+def _is_server_responsive(host: str = "127.0.0.1", port: int = 5001) -> bool:
+    """Check if a server is actually responding on the given port."""
+    import socket
+    try:
+        with socket.create_connection((host, port), timeout=0.5):
+            return True
+    except (OSError, ConnectionRefusedError, TimeoutError):
+        return False
+
+
 def get_or_create_server() -> SimulationServer:
     """Get or create the simulation server singleton.
 
@@ -118,9 +128,14 @@ def get_or_create_server() -> SimulationServer:
     """
     global _simulation_server, _server_thread
 
-    # Return existing server if it's running
+    # Return existing server if it's running AND responsive
     if _simulation_server is not None and _simulation_server.is_running:
+        # Verify the server is actually responding (not a stale flag)
+        if _is_server_responsive():
             return _simulation_server
+        # Server flag says running but it's not responsive - clean up
+        _simulation_server = None
+        _server_thread = None
 
     # Create new server
     server = SimulationServer(

src/py_dvt_ate/instruments/transport/server.py

@@ -138,6 +138,7 @@ class InstrumentServer:
                 handler,
                 self._host,
                 port,
+                reuse_address=True,
             )
             self._servers.append(server)
             logger.info(