TUI shell + ftxui via FetchContent.

- ftxui v6.1.9 fetched at configure time; vendored .a + headers dropped.
- TUI: visualisation area on top, input prompt at the bottom; ↑/↓ history,
  Tab completion (commands + file paths), Esc cancels prompts. History is
  persisted (XDG on Linux, %LOCALAPPDATA% on Windows when ported).
- Command registry: `new`, `load`, `search`, `clear`, `help`, `quit`/`exit`.
  Inline params or interactive prompts; either way the canonical inline
  form is what gets stored in history.
- `search`: second full-screen mode with module + parts/signals menus and
  a live-filtered list; Tab cycles focus, Esc returns to the main shell.
- Domain: `System::modules()` accessor + `SystemElementContainer::size()`
  to support load summary + search.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/tui/tui.hpp

@@ -0,0 +1,75 @@
+#ifndef _TUI_HPP_
+#define _TUI_HPP_
+
+#include <deque>
+#include <functional>
+#include <map>
+#include <memory>
+#include <string>
+#include <vector>
+
+class System;
+
+class Tui {
+    struct Prompt {
+        std::string question;
+        std::function<void(const std::string &)> on_answer;
+        bool path_completion = false;
+    };
+
+    struct CommandSpec {
+        struct Param {
+            std::string name;
+            bool path_completion = false;
+        };
+        std::vector<Param> params;
+        std::function<void(const std::vector<std::string> &)> action;
+    };
+
+    std::vector<std::string> history;
+    std::vector<std::string> output;
+    std::string input;
+    int cursor_pos;
+    int history_idx;
+    bool quit;
+
+    std::unique_ptr<System> sys;
+    std::deque<Prompt> pending;
+    std::map<std::string, CommandSpec> commands;
+
+    int screen_idx;
+    std::vector<std::string> search_modules;
+    std::vector<std::string> search_types;
+    int search_module_idx;
+    int search_type_idx;
+    int search_focus_idx;
+    std::string search_query;
+
+public:
+    Tui();
+    ~Tui();
+    void Run();
+
+private:
+    void RegisterCommands();
+    void Submit();
+    void Dispatch(const std::string &raw);
+    void Finalize(const std::string &name,
+                  const CommandSpec &spec,
+                  const std::vector<std::string> &args);
+
+    void HistoryUp();
+    void HistoryDown();
+    void CancelPending();
+    void Print(const std::string &line);
+
+    void CompleteCommand();
+    void CompletePath();
+
+    void LoadHistory();
+    void AppendHistory(const std::string &cmd);
+
+    static std::vector<std::string> Tokenize(const std::string &s);
+};
+
+#endif // _TUI_HPP_