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/sk6812.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 SK6812Device(BaseDevice):
@@ -24,5 +25,5 @@ class SK6812Device(BaseDevice):
         return bytes(buf)
 
     def encode_animation_cmd(self, animation: str, params: dict) -> bytes:
-        """Stub — Phase 3 implements real animation command encoding."""
+        """Encode animation command as 16-byte 0xAC LightSync packet."""
-        return b""
+        return _encode_cmd(animation, params)

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."""
+        """Encode animation command as 16-byte 0xAC LightSync packet."""
-        return b""
+        return _encode_cmd(animation, params)

lightsync/protocol/udp_sender.py

@@ -0,0 +1,63 @@
+"""Async UDP transport wrapper for sending LED packets to devices.
+
+Provides a single shared socket that can sendto() multiple device addresses.
+One socket handles all devices — UDP is connectionless, no socket pool needed.
+
+Usage:
+    sender = UDPSender()
+    await sender.start()              # Open socket at app startup
+    sender.send(payload, ip, port)    # Fire-and-forget from async context
+    await sender.stop()               # Close socket at app shutdown
+"""
+import asyncio
+
+
+class _NullProtocol(asyncio.DatagramProtocol):
+    """Minimal datagram protocol — we only send, never receive on this socket."""
+
+    def error_received(self, exc: Exception) -> None:
+        # UDP send errors are non-fatal for LED strip delivery.
+        # Network hiccups should not crash the show.
+        pass
+
+
+class UDPSender:
+    """Single shared UDP socket for sending LED frames and animation commands.
+
+    Thread safety: sendto() is NOT thread-safe across OS thread boundaries.
+    Always call send() from within the asyncio event loop. If bridging from
+    sync code, use loop.call_soon_threadsafe().
+    """
+
+    def __init__(self) -> None:
+        self._transport: asyncio.DatagramTransport | None = None
+
+    async def start(self) -> None:
+        """Open the UDP socket. Call once during FastAPI lifespan startup."""
+        loop = asyncio.get_running_loop()
+        self._transport, _ = await loop.create_datagram_endpoint(
+            _NullProtocol,
+            local_addr=("0.0.0.0", 0),  # OS assigns ephemeral source port
+        )
+
+    def send(self, payload: bytes, ip: str, port: int) -> None:
+        """Send payload to (ip, port). Fire-and-forget — no acknowledgement.
+
+        Args:
+            payload: Raw bytes to send (DRGB/DRGBW/DNRGB/animation command).
+            ip: Destination IP address string.
+            port: Destination UDP port (WLED default: 21324).
+        """
+        if self._transport:
+            self._transport.sendto(payload, (ip, port))
+
+    async def stop(self) -> None:
+        """Close the UDP socket. Call during FastAPI lifespan shutdown."""
+        if self._transport:
+            self._transport.close()
+            self._transport = None
+
+    @property
+    def is_running(self) -> bool:
+        """True if the socket is open and ready to send."""
+        return self._transport is not None