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-open.md

@@ -0,0 +1,23 @@
+---
+id: udp-open
+name: Udp.open
+category: udp
+kind: namespace-method
+tokens: Udp.open
+sig: Udp.open(port) -> int
+tip: Open a socket on a port.
+order: 0
+ns: Udp
+member: open
+---
+
+Opens a non-blocking IPv4 socket bound to <code>port</code> on every interface and returns its handle, or <code>0</code> when it could not. Port <code>0</code> lets the system pick a free one; <a href="udp-port.html"><code>Udp.port</code></a> says which.
+
+```ludic
+program Demo {
+  entry {
+    let s = Udp.open(port: 0)
+    print(s > 0)
+  }
+}
+```