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/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`).