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-resolve.md

@@ -0,0 +1,22 @@
+---
+id: udp-resolve
+name: Udp.resolve
+category: udp
+kind: namespace-method
+tokens: Udp.resolve
+sig: Udp.resolve(name) -> int
+tip: Look a host name up.
+order: 7
+ns: Udp
+member: resolve
+---
+
+Returns the first IPv4 address of a host name, or <code>0</code>. It waits for the system's resolver, so call it outside the frame loop.
+
+```ludic
+program Demo {
+  entry {
+    print(Udp.ip_text(ip: Udp.resolve(name: "localhost")))
+  }
+}
+```