Fix Windows networking bugs and connection metadata mismatch

- Use closesocket() instead of close() for sockets on Windows (lib/tcp.c)
- Call WSAStartup in tcp_init so outbound connections work (lib/tcp.c)
- Check WSAGetLastError() instead of errno for recv/send on Windows (lib/tcp.c)
- Use WSAEWOULDBLOCK instead of EINPROGRESS for non-blocking connect (lib/tcp.c)
- Add NULL guard in tcp_write for stale connection handles (lib/tcp.c)
- Fix ConnMetadata/TCP_Conn index mismatch in get_next_message: use
  event.handle.idx directly instead of searching for a free metadata
  slot, which could diverge when stale entries remain from failed
  outbound connections (lib/message.c)
- Initialize num_senders to 0 and skip unused metadata in
  find_conn_by_target (lib/message.c)
- Clear ready flag in http_server_next_request to prevent infinite
  loop (lib/http_server.c)
- Add CRT invalid parameter handler for the HTTP proxy (src/main.c)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/http_proxy.c

@@ -272,4 +272,4 @@ int http_proxy_free(void *state)
     http_server_free(&proxy->http_server);
     free(proxy->opers);
     return 0;
-}
+}

src/main.c

@@ -319,6 +319,16 @@ int main(int argc, char **argv)
 
 #ifdef _WIN32
 #include <winsock2.h>
+#include <stdlib.h>
+#include <stdio.h>
+static void invalid_param_handler(const wchar_t *expr, const wchar_t *func,
+    const wchar_t *file, unsigned int line, uintptr_t reserved)
+{
+    (void)expr; (void)func; (void)file; (void)line; (void)reserved;
+    fprintf(stderr, "[CRT] Invalid parameter in %ls (%ls:%u)\n",
+        func ? func : L"?", file ? file : L"?", line);
+    fflush(stderr);
+}
 #else
 #include <poll.h>
 #endif
@@ -329,6 +339,9 @@ int main(int argc, char **argv)
 
 int main(int argc, char **argv)
 {
+#ifdef _WIN32
+    _set_invalid_parameter_handler(invalid_param_handler);
+#endif
     int ret;
 
     HTTPProxy state;