[zgh419566]

What

Add UdpConnection::sendTo(string $ip, int $port, mixed $data, bool $raw = false) for sending to an arbitrary destination.

Add Worker::getUdpSender(): ?UdpConnection to obtain a reusable sender bound to the listening socket.

Default $remoteAddress to '' in UdpConnection::__construct() to allow creating a sender-only connection.

Add an udp server example and update README.

Why
UdpConnection::send() only replies to the address of the incoming packet. Protocols like SIP/GB28181 require sending requests from the listening port (so the source port matches), validated by the peer. Currently, users must manually create a second socket. sendTo() and getUdpSender() fill this gap elegantly, keeping the logic inside the official UdpConnection without touching send()'s signature.

Why not modify send()
Adding a third parameter to send() would break signature compatibility for AsyncUdpConnection subclasses. A dedicated sendTo() is also clearer: send() replies, sendTo() targets. Placing it on UdpConnection makes the UDP-only nature obvious, as TcpConnection has no such concept.

Backward compatibility
Zero breakage. send() is untouched. The constructor change only adds a default value.

[zgh419566]

This PR adds the ability for a UDP worker to proactively send packets on its listening port, without waiting for an incoming packet. Two new methods are added:

UdpConnection::sendTo(string $ip, int $port, mixed $data, bool $raw = false): bool
Sends a datagram to an arbitrary destination using the connection's socket as the source.

Worker::getUdpSender(): ?UdpConnection
Returns a reusable sender bound to the worker's listening socket, for use in onWorkerStart, timers, and channel callbacks.

Additionally, UdpConnection::__construct() now defaults $remoteAddress to '', so a sender-only connection can be created without a remote address. A new example in README snippet are included.

本 PR 让 UDP Worker 可以用自己的监听端口主动发数据，而不必等收到包再回复。新增两个方法：

UdpConnection::sendTo(string $ip, int $port, mixed $data, bool $raw = false): bool
用当前连接的 socket 作为源，发往任意目标地址。

Worker::getUdpSender(): ?UdpConnection
返回一个绑定到监听 socket 的可复用发送器，方便在 onWorkerStart、Timer、channel 回调里使用。

此外，UdpConnection::__construct() 的 $remoteAddress 参数加了默认值 ''，这样"只用来发送"的连接不需要远端地址就能构造。同时补充了示例的 README 片段。