Implement TCP client handling

Add async client connection handling with:
- Multiple concurrent connections per port
- Line-based SCPI protocol (newline terminated)
- start(), stop(), and serve_forever() methods
- Proper connection lifecycle and error handling

src/py_dvt_ate/simulation/tcp_server.py

@@ -14,6 +14,9 @@ from typing import TYPE_CHECKING
 if TYPE_CHECKING:
     from py_dvt_ate.simulation.virtual.base import BaseInstrument
 
+# Re-export for type checking - actual import happens at runtime via registration
+__all__ = ["InstrumentServer"]
+
 logger = logging.getLogger(__name__)
 
 
@@ -88,3 +91,122 @@ class InstrumentServer:
     def registered_ports(self) -> list[int]:
         """Get list of registered port numbers."""
         return list(self._instruments.keys())
+
+    async def start(self) -> None:
+        """Start the server and begin listening on all registered ports.
+
+        Creates a TCP server for each registered instrument port.
+
+        Raises:
+            RuntimeError: If server is already running or no instruments registered.
+        """
+        if self._running:
+            raise RuntimeError("Server is already running")
+
+        if not self._instruments:
+            raise RuntimeError("No instruments registered")
+
+        self._running = True
+
+        for port, instrument in self._instruments.items():
+            server = await asyncio.start_server(
+                lambda r, w, inst=instrument, p=port: self._handle_client(r, w, inst, p),
+                self._host,
+                port,
+            )
+            self._servers.append(server)
+            logger.info(
+                "Started server for %s on %s:%d",
+                instrument.__class__.__name__,
+                self._host,
+                port,
+            )
+
+    async def stop(self) -> None:
+        """Stop the server and close all connections."""
+        if not self._running:
+            return
+
+        for server in self._servers:
+            server.close()
+            await server.wait_closed()
+
+        self._servers.clear()
+        self._running = False
+        logger.info("Server stopped")
+
+    async def serve_forever(self) -> None:
+        """Start the server and run until cancelled.
+
+        This is a convenience method that starts the server and blocks
+        until the server is stopped or cancelled.
+        """
+        await self.start()
+        try:
+            # Keep running until cancelled
+            await asyncio.gather(
+                *[server.serve_forever() for server in self._servers]
+            )
+        finally:
+            await self.stop()
+
+    async def _handle_client(
+        self,
+        reader: asyncio.StreamReader,
+        writer: asyncio.StreamWriter,
+        instrument: BaseInstrument,
+        port: int,
+    ) -> None:
+        """Handle a client connection.
+
+        Reads SCPI commands (newline-terminated), processes them through
+        the instrument, and sends back responses (newline-terminated).
+
+        Args:
+            reader: Stream reader for incoming data.
+            writer: Stream writer for outgoing data.
+            instrument: The instrument to process commands.
+            port: Port number for logging.
+        """
+        addr = writer.get_extra_info("peername")
+        logger.info("Client connected to port %d from %s", port, addr)
+
+        try:
+            while True:
+                # Read until newline (SCPI line terminator)
+                data = await reader.readline()
+
+                if not data:
+                    # Client disconnected
+                    break
+
+                # Decode and strip whitespace
+                command = data.decode("utf-8").strip()
+
+                if not command:
+                    continue
+
+                logger.debug("Port %d received: %s", port, command)
+
+                # Process command through instrument
+                response = instrument.process(command)
+
+                # Send response with newline terminator
+                if response:
+                    writer.write(f"{response}\n".encode("utf-8"))
+                    await writer.drain()
+                    logger.debug("Port %d sent: %s", port, response)
+
+        except asyncio.CancelledError:
+            logger.debug("Client handler cancelled for port %d", port)
+        except ConnectionResetError:
+            logger.debug("Client connection reset on port %d", port)
+        except Exception as e:
+            logger.error("Error handling client on port %d: %s", port, e)
+        finally:
+            writer.close()
+            try:
+                await writer.wait_closed()
+            except Exception:
+                pass
+            logger.info("Client disconnected from port %d", port)