build: factor per-frontend CMake into essim_add_frontend()

Every frontend repeated the same target wiring (glob sources minus main.cpp →
essim_<name> lib linking essim_core; essim exe from main.cpp linking the lib +
essim_frontend; RUNTIME_OUTPUT_DIRECTORY=build/). Move it into a reusable
helper cmake/EssimFrontend.cmake::essim_add_frontend(name LIBS ...), included
once at the top level.

The tui CMakeLists now just fetches FTXUI and calls
essim_add_frontend(tui LIBS ftxui::screen ftxui::dom ftxui::component). A new
frontend's CMakeLists is its toolkit setup + one call. No behaviour change;
binary stays ./build/essim, tests green.

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

CMakeLists.txt

@@ -11,6 +11,10 @@ project(essim
 
 include(FetchContent)
 
+# Shared CMake helpers (essim_add_frontend — per-frontend target boilerplate).
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
+include(EssimFrontend)
+
 # ----------------------------------------------------------------- core deps
 # libbsdl — standalone BSDL parser (LGPL-2.1), dynamically linked (EUPL-1.2,
 # which the LGPL permits). Override its path with -DBSDL_DIR=...