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>

CMakeLists.txt

@@ -48,7 +48,7 @@ target_link_libraries(essim_core
 # src/frontends/gui/ and configure with -DESSIM_FRONTEND=gui.
 set(ESSIM_FRONTEND "tui" CACHE STRING
     "Frontend to build: a directory name under src/frontends/, or 'none'")
-set_property(CACHE ESSIM_FRONTEND PROPERTY STRINGS tui none)
+set_property(CACHE ESSIM_FRONTEND PROPERTY STRINGS tui wx none)
 
 if(ESSIM_FRONTEND STREQUAL "none")
   message(STATUS "essim: ESSIM_FRONTEND=none — core + tests only (no frontend, no GUI toolkit)")