doc: README — explicit wx GUI build command

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

README.md

@@ -22,6 +22,17 @@ library (domain + importers + operations); the `essim` binary comes from a
 core + tests only, with no GUI toolkit fetched. Architecture in
 [`DESIGN.md`](DESIGN.md).
 
+The quick start above builds the **tui** (terminal) frontend. For the
+**wxWidgets GUI** instead (needs wxWidgets ≥ 3.2, see *Dependencies*):
+
+```sh
+cmake -S . -B build -DESSIM_FRONTEND=wx
+cmake --build build -j
+./build/essim
+```
+
+Switching `-DESSIM_FRONTEND` on an existing `build/` reconfigures it in place.
+
 Inside the shell, type `help` for the live command list — or read the
 auto-generated reference at [`doc/user/commands.md`](doc/user/commands.md).
 A worked bring-up script is at [`test/system.essim`](test/system.essim);