mplex: client, cli, plexer flow controllers and validators, wiring doc, differential tests

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

taicpp/tests/mplex_cli_main.cpp

@@ -0,0 +1,30 @@
+// Test artifact: runs the C++ mplex client/server as a standalone binary, mirroring what
+// tai.py main does for `--mplex [COMMAND...]` and `--plexer-wrap`. Not part of the detz build.
+#include <cstdlib>
+#include <cstring>
+#include <string>
+#include <vector>
+
+#include "mplex_client.h"
+
+int main(int argc, char** argv) {
+  using namespace detz::mplex;
+  std::string home;
+  const char* env_home = std::getenv("TAI_HOME");
+  if (env_home && *env_home) {
+    home = env_home;
+  } else {
+    const char* user_home = std::getenv("HOME");
+    home = std::string(user_home ? user_home : "") + "/.tai";
+  }
+  if (argc >= 2 && std::strcmp(argv[1], "--mplex") == 0) {
+    std::vector<std::string> words(argv + 2, argv + argc);
+    return MplexCli::main(home, words);
+  }
+  if (argc >= 2 && std::strcmp(argv[1], "--plexer-wrap") == 0) {
+    NativePlexerBackend backend(home, "default");
+    return backend.run();
+  }
+  std::fputs("usage: mplex_cli --mplex [COMMAND...] | --plexer-wrap\n", stderr);
+  return 2;
+}