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>

selfhost/backend/emit_call.ludic

@@ -517,6 +517,21 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if (meth == "free")      { bare = "http_close";       push(labels, "handle") }
     if (meth == "parse")     { bare = "http_parse";       push(labels, "bytes"); push(labels, "len") }
   }
+  # Udp.* — polled IPv4 datagrams (runtime/native/udp.ludic + udp.ll / udp_win.ll).
+  # Out-of-band like Http.*: the transport under a game's own netcode.
+  if (ns == "Udp") {
+    if (meth == "open")      { bare = "udp_open";       push(labels, "port") }
+    if (meth == "port")      { bare = "udp_port";       push(labels, "socket") }
+    if (meth == "send")      { bare = "udp_send";       push(labels, "socket"); push(labels, "ip"); push(labels, "port"); push(labels, "bytes"); push(labels, "len") }
+    if (meth == "recv")      { bare = "udp_recv";       push(labels, "socket"); push(labels, "bytes"); push(labels, "cap") }
+    if (meth == "from_ip")   { bare = "udp_from_ip";    push(labels, "socket") }
+    if (meth == "from_port") { bare = "udp_from_port";  push(labels, "socket") }
+    if (meth == "close")     { bare = "udp_close";      push(labels, "socket") }
+    if (meth == "resolve")   { bare = "udp_resolve";    push(labels, "name") }
+    if (meth == "local_ip")  { bare = "udp_local_ip" }
+    if (meth == "ip")        { bare = "udp_ip";         push(labels, "text") }
+    if (meth == "ip_text")   { bare = "udp_ip_text";    push(labels, "ip") }
+  }
   # Phase 3: the bare reflection / networking / process builtins, namespaced.
   # Each is a pure alias — the callee is rewritten to the bare name below.
   if (ns == "World") {

selfhost/frontend/parse.ludic

@@ -252,6 +252,8 @@ function p_postfix() -> Node {
       if e.a.kind == E_ID and e.a.s == "Audio" { g_uses_audio = true }
       # Http.* (#6) — any Http method splices the HTTP client runtime.
       if e.a.kind == E_ID and e.a.s == "Http" { g_uses_http = true }
+      # Udp.* — any Udp method splices the datagram runtime (and links udp.ll / udp_win.ll).
+      if e.a.kind == E_ID and e.a.s == "Udp" { g_uses_udp = true }
       # Gl.* — any Gl method splices the OpenGL runtime (and links the GL backend).
       if e.a.kind == E_ID and e.a.s == "Gl" { g_uses_gl = true }
       # Vk.* — any Vk method splices the Vulkan runtime (and links its loader)
@@ -677,6 +679,7 @@ var g_uses_tween_rt: bool = false  # Tween.to/chain/delay/… (#48) -> splice tw
 var g_uses_fx: bool = false       # Fx.sparks/number/clear -> splice fx.ludic; fx_tick each Update, fx_draw each Render
 var g_uses_audio: bool = false    # Audio.* (#22) -> splice audio.ludic; a windowed build also links audio.ll + AVFoundation
 var g_uses_http:  bool = false    # Http.* (#6) -> splice http.ludic; links http.ll + Foundation (macOS)
+var g_uses_udp:   bool = false    # Udp.* -> splice udp.ludic; links udp.ll (macOS) / udp_win.ll + ws2_32
 var g_uses_gl:    bool = false    # Gl.* -> splice gl.ludic (+ generated gl_api.ludic); links gl.ll + gl_thunks.ll + OpenGL
 var g_uses_vk:    bool = false    # Vk.* -> splice vk.ludic (+ generated vk_api.ludic); links vk_thunks.ll + the platform loader
 # issue #64: functions marked @System(Phase) in a prebuilt binary module — the
@@ -1187,6 +1190,13 @@ function maybe_splice_runtime() -> void {
     do_import("runtime/native/http.ludic")
     cur_dir = saved
   }
+  # Udp.*: splice the datagram library. The sockets are native (udp.ll / udp_win.ll, bound
+  # by `extern function` in udp.ludic); the address helpers are pure Ludic.
+  if g_uses_udp {
+    cur_dir = ""
+    do_import("runtime/native/udp.ludic")
+    cur_dir = saved
+  }
   # Gl.*: splice the OpenGL surface (gl.ludic + the generated gl_api.ludic). The
   # native calls are the linked GL entry points themselves; the window attach is
   # is_windowed()-guarded, so a headless build renders into an offscreen context.

selfhost/main.ludic

@@ -273,6 +273,12 @@ entry {
     if g_target_win { cmd = `{cmd} {join_path(home, "runtime/native/threads_win.ll")}` }
     else { cmd = `{cmd} {join_path(home, "runtime/native/threads.ll")}` }
   }
+  # Udp.* links the socket layer: BSD sockets from libSystem on macOS, Winsock (ws2_32) on
+  # Windows.
+  if g_uses_udp {
+    if g_target_win { cmd = `{cmd} {join_path(home, "runtime/native/udp_win.ll")} -lws2_32` }
+    else { cmd = `{cmd} {join_path(home, "runtime/native/udp.ll")}` }
+  }
   if g_uses_http {
     if g_target_win {
       let httpw = join_path(home, "runtime/native/http_win.ll")