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-close.md

@@ -0,0 +1,23 @@
+---
+id: udp-close
+name: Udp.close
+category: udp
+kind: namespace-method
+tokens: Udp.close
+sig: Udp.close(socket) -> void
+tip: Close a socket.
+order: 6
+ns: Udp
+member: close
+---
+
+Closes the socket and frees its handle.
+
+```ludic
+program Demo {
+  entry {
+    let s = Udp.open(port: 0)
+    Udp.close(socket: s)
+  }
+}
+```