cli: faithful argparse port (single flag table, HelpFormatter, textwrap, abbreviations, usage on error)

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

taicpp/tests/cli_main.cpp

@@ -0,0 +1,14 @@
+// Minimal detz entry point over the cli module only, so tools/parity_diff.py can run the
+// parse-only cases (tests/diff/cases/cli_generated.json) without the full binary.
+// It mirrors the first lines of main.cpp (error, version) and then Command::run.
+#include <cstdio>
+#include <string>
+#include "../src/cli/cli.h"
+#include "version.hpp"
+int main(int argc, char** argv) {
+    detz::cli::Parser p;
+    auto cmd = p.parse(argc, argv);
+    if (!cmd.opts.error.empty()) { std::fputs((cmd.opts.error + "\n").c_str(), stderr); return 2; }
+    if (cmd.opts.version) { std::puts(("tai " + std::string(detz::kVersion)).c_str()); return 0; }
+    return cmd.run();
+}