tui: add --batch mode to run a script and print output headless

BootDispatch already runs --restore/--source synchronously before the TUI
starts (Source takes its headless drain branch when no screen is attached),
so the console buffer is complete by then. New --batch flag dumps that buffer
(Tui::DumpOutput) to stdout and exits without launching the TUI — enabling
scripted/CI runs and verify output capture (e.g. essim --batch --source s).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main.cpp

@@ -8,7 +8,7 @@ namespace {
 
 void print_usage(const char *prog) {
     std::cerr <<
-        "usage: " << prog << " [--source FILE] [--restore FILE]\n"
+        "usage: " << prog << " [--batch] [--source FILE] [--restore FILE]\n"
         "       " << prog << " --commands-md [FILE]\n"
         "       " << prog << " --help\n"
         "  (no args)             launch the TUI on an empty system.\n"
@@ -19,6 +19,8 @@ void print_usage(const char *prog) {
         "                        FILE (same as the `restore` command).\n"
         "                        Combine with --source to layer a script on\n"
         "                        top of a restored snapshot.\n"
+        "  --batch               run --restore/--source, print the console\n"
+        "                        output to stdout, and exit without the TUI.\n"
         "  --commands-md [FILE]  dump the command registry as Markdown.\n"
         "                        With FILE: write there. Without: stdout.\n"
         "                        (Used by `cmake --build build --target doc`.)\n"
@@ -30,6 +32,7 @@ void print_usage(const char *prog) {
 int main(int argc, char **argv) {
     std::string boot_restore;
     std::string boot_source;
+    bool batch = false;
 
     for (int i = 1; i < argc; ++i) {
         std::string a = argv[i];
@@ -63,6 +66,10 @@ int main(int argc, char **argv) {
             boot_restore = argv[++i];
             continue;
         }
+        if (a == "--batch") {
+            batch = true;
+            continue;
+        }
         if (a == "--help" || a == "-h") {
             print_usage(argv[0]);
             return 0;
@@ -78,6 +85,14 @@ int main(int argc, char **argv) {
     // snapshot, then re-run a small script that adds a new card").
     if (!boot_restore.empty()) tui.BootDispatch("restore " + boot_restore);
     if (!boot_source.empty())  tui.BootDispatch("source "  + boot_source);
+
+    // Batch mode: the boot dispatch already ran synchronously (no screen yet),
+    // so the console output is complete. Print it and exit without the TUI.
+    if (batch) {
+        tui.DumpOutput(std::cout);
+        return 0;
+    }
+
     tui.Run();
     return 0;
 }

src/tui/shell.cpp

@@ -36,6 +36,11 @@ void Tui::Print(const std::string &line) {
     scroll_offset = 0;  // any new line snaps the view back to the tail
 }
 
+void Tui::DumpOutput(std::ostream &out) const {
+    for (const std::string &line : output)
+        out << line << "\n";
+}
+
 void Tui::HistoryUp() {
     if (history.empty()) return;
     if (history_idx == -1) history_idx = (int)history.size() - 1;

src/tui/tui.hpp

@@ -197,6 +197,9 @@ public:
     ~Tui();
     void Run();
     void DumpCommandsMd(std::ostream &out) const;
+    // Write the accumulated console output to `out`. Used by batch mode to
+    // surface a script's output without starting the TUI.
+    void DumpOutput(std::ostream &out) const;
 
     // Boot-time hook: dispatch a single command exactly as if the user
     // typed it (e.g. `restore foo.essim` or `source bring-up.essim`).