Add example external client for testing and documentation

Provide a minimal working example of an external client that users can:
- Test with: pi-pianoteq --client example_client:ExampleClient
- Use as a template for custom client development
- Reference when following the custom client documentation

The example demonstrates all required Client abstract methods with
simple console output, making it easy to understand the client lifecycle.

Author: claude

example_client.py

@@ -0,0 +1,62 @@
+"""
+Example minimal external client for Pi-Pianoteq.
+
+This demonstrates the minimum required implementation for a custom client.
+You can test it with: pi-pianoteq --client example_client:ExampleClient
+"""
+from typing import Optional
+import logging
+
+from pi_pianoteq.client.client import Client
+from pi_pianoteq.client.client_api import ClientApi
+
+
+class ExampleClient(Client):
+    """
+    Minimal example client that prints to console.
+
+    Demonstrates the basic structure for a custom client.
+    """
+
+    def __init__(self, api: Optional[ClientApi]):
+        """Initialize the client in loading mode (api=None)"""
+        super().__init__(api)
+        print("ExampleClient initialized")
+
+    def set_api(self, api: ClientApi):
+        """Called when the API becomes available"""
+        self.api = api
+        print(f"ExampleClient: API ready with {len(api.get_instruments())} instruments")
+
+    def show_loading_message(self, message: str):
+        """Display loading messages during startup"""
+        print(f"[LOADING] {message}")
+
+    def start(self):
+        """Main client loop - called after set_api()"""
+        print("\nExampleClient started!")
+        print("=" * 60)
+
+        # Show current state
+        instrument = self.api.get_current_instrument()
+        preset = self.api.get_current_preset()
+        print(f"Current: {instrument.name} - {preset.name}")
+
+        # List available instruments
+        print(f"\nAvailable instruments ({len(self.api.get_instruments())}):")
+        for i, instr in enumerate(self.api.get_instruments(), 1):
+            print(f"  {i}. {instr.name}")
+
+        print("\nPress Ctrl+C to exit")
+
+        # Simple loop - just wait for interrupt
+        try:
+            import time
+            while True:
+                time.sleep(1)
+        except KeyboardInterrupt:
+            print("\nExampleClient shutting down...")
+
+    def get_logging_handler(self) -> Optional[logging.Handler]:
+        """Return None to use default stdout/stderr logging"""
+        return None