feat(udp): Udp.* - polled IPv4 datagrams on macOS and Windows

Udp.open/port/send/recv/from_ip/from_port/close/resolve/local_ip/ip/ip_text, native
BSD sockets (udp.ll) and Winsock (udp_win.ll, -lws2_32), linked only when a program
uses Udp.*; example, docs and tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/language/udp/udp-send.md

@@ -0,0 +1,25 @@
+---
+id: udp-send
+name: Udp.send
+category: udp
+kind: namespace-method
+tokens: Udp.send
+sig: Udp.send(socket, ip, port, bytes, len) -> int
+tip: Send one datagram.
+order: 2
+ns: Udp
+member: send
+---
+
+Sends the first <code>len</code> bytes of <code>bytes</code> to <code>ip</code>:<code>port</code> as one datagram and returns the number of bytes sent, or <code>-1</code>.
+
+```ludic
+program Demo {
+  entry {
+    let s = Udp.open(port: 0)
+    let buf = Memory.bytes(4)
+    Memory.poke(buf, 0, 1)
+    print(Udp.send(socket: s, ip: Udp.ip(text: "127.0.0.1"), port: Udp.port(socket: s), bytes: buf, len: 1))
+  }
+}
+```