fix(dashboard): use module-level singleton to prevent port conflicts on refresh

When Streamlit refreshes/reruns, session state is lost but the old
simulation server thread keeps running on ports 5001-5003. This caused
"address already in use" errors when trying to start a new server.

Solution: Use a module-level singleton for the simulation server that
persists across Streamlit reruns. The get_or_create_server() function
checks if a server is already running before creating a new one.

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

src/py_dvt_ate/app/dashboard/app.py

@@ -28,6 +28,12 @@ from py_dvt_ate.tests.thermal.tempco import TempCoTest
 # Thread pool for background test execution
 _test_executor = ThreadPoolExecutor(max_workers=1, thread_name_prefix="test_runner")
 
+# Module-level singleton for simulation server
+# This ensures only one server instance exists across all Streamlit reruns,
+# preventing "address already in use" errors when the page refreshes
+_simulation_server: SimulationServer | None = None
+_server_thread: threading.Thread | None = None
+
 # Idle pause configuration
 # Physics engine pauses after IDLE_PAUSE_SECONDS of no activity (default: 30s)
 IDLE_PAUSE_SECONDS = int(os.environ.get("IDLE_PAUSE_SECONDS", "30"))
@@ -101,12 +107,22 @@ class TestProgress:
         return time.time() - self.started_at
 
 
-def start_embedded_server() -> tuple[SimulationServer, threading.Thread]:
-    """Start an embedded simulation server in a background thread.
+def get_or_create_server() -> SimulationServer:
+    """Get or create the simulation server singleton.
+
+    Uses module-level singleton to ensure only one server instance exists
+    across all Streamlit reruns, preventing "address already in use" errors.
 
     Returns:
-        Tuple of (server instance, thread running the server).
+        The simulation server instance.
     """
+    global _simulation_server, _server_thread
+
+    # Return existing server if it's running
+    if _simulation_server is not None and _simulation_server.is_running:
+        return _simulation_server
+
+    # Create new server
     server = SimulationServer(
         ServerConfig(
             host="127.0.0.1",
@@ -151,27 +167,34 @@ def start_embedded_server() -> tuple[SimulationServer, threading.Thread]:
     if server_error:
         st.error(f"Server startup error: {server_error[0]}")
 
-    return server, thread
+    # Store in module-level singleton
+    _simulation_server = server
+    _server_thread = thread
+
+    return server
 
 
 def _update_activity() -> None:
     """Update activity timestamp and resume physics if paused."""
-    global _last_activity_time
+    global _last_activity_time, _simulation_server
     _last_activity_time = time.time()
 
-    # Resume physics if it was paused
-    if "server" in st.session_state:
-        server = st.session_state.server
-        if server.paused:
+    # Resume physics if it was paused (use singleton as fallback)
+    server = st.session_state.get("server") or _simulation_server
+    if server is not None and server.paused:
         server.paused = False
         print("Physics engine resumed (user activity detected)")
 
 
 def init_session_state() -> None:
     """Initialise Streamlit session state."""
-    if "server" not in st.session_state:
+    global _simulation_server
+
+    # Get or create the server singleton (survives Streamlit reruns)
+    if "server" not in st.session_state or st.session_state.server is None:
         with st.spinner("Starting simulation server..."):
-            st.session_state.server, st.session_state.server_thread = start_embedded_server()
+            server = get_or_create_server()
+            st.session_state.server = server
 
         # Verify server started correctly
         if st.session_state.server.physics_engine is None:
@@ -181,15 +204,17 @@ def init_session_state() -> None:
         # Start idle checker to pause physics when no one's viewing
         _start_idle_checker(st.session_state.server)
 
-        # Register cleanup
+        # Register cleanup (only once, for the singleton)
         def cleanup() -> None:
-            if hasattr(st.session_state, "server") and st.session_state.server is not None:
+            global _simulation_server
+            if _simulation_server is not None:
                 loop = asyncio.new_event_loop()
                 try:
-                    loop.run_until_complete(st.session_state.server.stop())
+                    loop.run_until_complete(_simulation_server.stop())
                 except Exception:
                     pass
                 loop.close()
+                _simulation_server = None
         atexit.register(cleanup)
 
     if "instruments" not in st.session_state: