docs: README + DESIGN for the wx frontend and essim_add_frontend

Document the wxWidgets GUI frontend (wx dependency + -DESSIM_FRONTEND=wx in
README; layout tree + architecture note in DESIGN) and the factored
essim_add_frontend() per-frontend CMake helper.

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

README.md

@@ -51,10 +51,17 @@ Step-by-step walkthroughs for both the batch and TUI workflows are in
 - Fetched automatically via `FetchContent` (nothing to install): **FTXUI**
   v6.1.9 — only when building the **tui** frontend — and **doctest** v2.4.11
   for the tests.
+- **wxWidgets** (≥ 3.2) — only for the **wx** GUI frontend
+  (`-DESSIM_FRONTEND=wx`). Install the development package:
+  - Debian/Ubuntu — `sudo apt install libwxgtk3.2-dev`
+  - Arch — `sudo pacman -S wxwidgets-gtk3`
+  - Fedora — `sudo dnf install wxGTK-devel`
 - Optional, only for the `doc` target: **doxygen** and **python3**.
 
-libzip, pugixml and libbsdl are the **core** dependencies; FTXUI belongs to the
-tui frontend, so a `-DESSIM_FRONTEND=none` build needs none of it.
+libzip, pugixml and libbsdl are the **core** dependencies; each frontend pulls
+its own toolkit (FTXUI for tui, wxWidgets for wx), so a `-DESSIM_FRONTEND=none`
+build needs neither. Pick a GUI/TUI with `-DESSIM_FRONTEND=tui|wx` (default
+`tui`).
 
 ## Tests