feat(03-01): UDP sender and device stub completion

- UDPSender: single shared asyncio DatagramTransport, start/stop lifecycle, fire-and-forget send()
- _NullProtocol: swallows OS send errors (UDP delivery is best-effort for LEDs)
- SK6812Device.encode_animation_cmd: replaces b'' stub with real 16-byte 0xAC packet
- WS2801Device.encode_animation_cmd: replaces b'' stub with real 16-byte 0xAC packet
- Both devices delegate to encode_animation_cmd from lightsync.protocol.animation_cmd

lightsync/devices/ws2801.py

@@ -1,5 +1,6 @@
 from lightsync.devices.base import BaseDevice
 from lightsync.models.device import DeviceConfig
+from lightsync.protocol.animation_cmd import encode_animation_cmd as _encode_cmd
 
 
 class WS2801Device(BaseDevice):
@@ -23,5 +24,5 @@ class WS2801Device(BaseDevice):
         return bytes(buf)
 
     def encode_animation_cmd(self, animation: str, params: dict) -> bytes:
-        """Stub — Phase 3 implements real animation command encoding."""
-        return b""
+        """Encode animation command as 16-byte 0xAC LightSync packet."""
+        return _encode_cmd(animation, params)