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-recv.md

@@ -0,0 +1,24 @@
+---
+id: udp-recv
+name: Udp.recv
+category: udp
+kind: namespace-method
+tokens: Udp.recv
+sig: Udp.recv(socket, bytes, cap) -> int
+tip: Read the next waiting datagram.
+order: 3
+ns: Udp
+member: recv
+---
+
+Copies the next waiting datagram, up to <code>cap</code> bytes, into <code>bytes</code> and returns its length, or <code>0</code> when nothing is waiting. It never blocks: call it in a loop each frame until it returns <code>0</code>. The sender is <a href="udp-from_ip.html"><code>Udp.from_ip</code></a> / <a href="udp-from_port.html"><code>from_port</code></a>.
+
+```ludic
+program Demo {
+  entry {
+    let s = Udp.open(port: 0)
+    let buf = Memory.bytes(64)
+    print(Udp.recv(socket: s, bytes: buf, cap: 64))
+  }
+}
+```