Add a wxWidgets GUI frontend (second frontend, proves the split).

A native wx GUI built entirely on essim_core via the Frontend interface — no
Tui reuse, no command shell. Demonstrates the core/frontends architecture by
adding a real second frontend:

  - WxFrontend : public Frontend — owns the System + a console buffer; handles
    boot headlessly (restore for --restore/--batch; honest note for source);
    Run() boots wx without a generated main (SetInstance + wxEntryStart/
    CallOnInit/OnRun) so the shared frontend_main stays in control.
  - EssimFrame (wxFrame) — menu-driven window: Load (app::load_module), Restore/
    Save (persist), Export (app::export_connections), Verify (app::verify),
    rendered into a model tree (modules → parts), an overview + verify-health
    panel, and a log. Each handler is a thin wrapper over a core/app op.
  - main.cpp: construct WxFrontend, call frontend_main — same 4 lines as tui.
  - CMakeLists.txt: find_package(wxWidgets) + essim_add_frontend(wx LIBS …);
    select with -DESSIM_FRONTEND=wx. ESSIM_FRONTEND gains wx in its STRINGS.

Set LC_CTYPE from the environment at GUI init so wxString decodes the UTF-8 in
narrow literals (em dash, ellipsis); LC_NUMERIC stays "C". .gitignore: build*/.

Needs libwxgtk3.2-dev. Verified: builds clean (wx 3.2.9 GTK3), window opens and
renders with no wx asserts; --commands-md/--restore/--batch behave headlessly.

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

src/frontends/wx/wx_frontend.hpp

@@ -0,0 +1,37 @@
+#ifndef _WX_FRONTEND_HPP_
+#define _WX_FRONTEND_HPP_
+
+#include "frontends/frontend.hpp"
+
+#include <memory>
+#include <string>
+
+class System;
+
+// wxWidgets GUI frontend. Implements the shared Frontend interface so the same
+// launcher (frontend_main) drives it: it owns the System and a console buffer,
+// handles boot commands headlessly (for --restore/--batch), and Run() opens the
+// wxWidgets window. The window itself (EssimFrame) drives essim_core / app::*
+// operations directly — no command shell, no TUI reuse.
+class WxFrontend : public Frontend {
+public:
+    WxFrontend();
+    ~WxFrontend() override;
+
+    // --- Frontend interface ---
+    void BootDispatch(const std::string &raw) override;
+    void DumpCommandsMd(std::ostream &out) const override;
+    void DumpOutput(std::ostream &out) const override;
+    void Run() override;
+
+    // --- used by the window (EssimFrame) ---
+    System *system() const { return sys_.get(); }
+    void set_system(System *fresh);   ///< take ownership (used by Restore)
+    void ensure_system();             ///< create an empty System if none yet
+
+private:
+    std::unique_ptr<System> sys_;
+    std::string output_;   ///< console buffer surfaced by DumpOutput (batch)
+};
+
+#endif  // _WX_FRONTEND_HPP_