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-ip.md

@@ -0,0 +1,22 @@
+---
+id: udp-ip
+name: Udp.ip
+category: udp
+kind: namespace-method
+tokens: Udp.ip
+sig: Udp.ip(text) -> int
+tip: Parse a dotted address.
+order: 9
+ns: Udp
+member: ip
+---
+
+Turns <code>"a.b.c.d"</code> into an address, or <code>0</code> when the text is not one.
+
+```ludic
+program Demo {
+  entry {
+    print(Udp.ip(text: "10.0.0.2"))
+  }
+}
+```