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>

DESIGN.md

@@ -78,6 +78,12 @@ src/
       commands_export.cpp  thin wrapper over app::export_connections
       screen_*.cpp         dashboard, connect, settype, explore, analyze, help, main
                            (console), palette, file dialog, error/confirm, sigtype modal
+    wx/                  -- wxWidgets GUI (builds libessim_wx + the `essim` binary)
+      CMakeLists.txt       find_package(wxWidgets) + essim_add_frontend(wx ...)
+      main.cpp             entry point: construct WxFrontend, call frontend_main
+      wx_frontend.{hpp,cpp}  WxFrontend : Frontend (owns System; boots wx in Run())
+      wx_frame.{hpp,cpp}   EssimFrame: menu/tree/overview/log over core + app::*
+cmake/EssimFrontend.cmake  -- essim_add_frontend(name LIBS ...) per-frontend wiring
 tests/                   -- core tests (link essim_core)
   tui/                   -- frontend tests (link essim_tui)
 doc/ , test/             -- docs; sample netlists + system.essim bring-up script
@@ -104,10 +110,13 @@ The hard rule: **`src/core/` never depends on a frontend** — no `#include
   `frontend_main(argc, argv, Frontend&)` (`frontends/frontend_main.cpp`, built
   into the toolkit-free `essim_frontend` lib) parses the CLI flags and drives the
   boot → batch/run flow through the interface; a frontend's `main()` is just
-  *construct the concrete Frontend, call `frontend_main`*. Add one by creating
-  `src/frontends/<name>/CMakeLists.txt` (build `essim_<name>` linking `essim_core`,
-  produce the `essim` binary linking `essim_frontend`) and configuring
-  `-DESSIM_FRONTEND=<name>`.
+  *construct the concrete Frontend, call `frontend_main`*. Two frontends ship
+  today: **tui** (FTXUI, default) and **wx** (a wxWidgets GUI, menu-driven over
+  `app::*`). Add another by creating `src/frontends/<name>/CMakeLists.txt` — its
+  toolkit setup (FetchContent / find_package) plus one call to
+  `essim_add_frontend(<name> LIBS …)` (the shared helper in
+  `cmake/EssimFrontend.cmake` that builds `essim_<name>` + the `essim` binary) —
+  and configuring `-DESSIM_FRONTEND=<name>`.
 
 Because the core links no toolkit, the suite links `essim_core` directly and
 `-DESSIM_FRONTEND=none` builds + tests the whole core with FTXUI never fetched.

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