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>

cmake/EssimFrontend.cmake

@@ -0,0 +1,30 @@
+# essim_add_frontend(<name> [LIBS <toolkit link targets>...])
+#
+# Builds the boilerplate shared by every frontend under src/frontends/<name>/:
+#   * a static library essim_<name> from every .cpp in the current directory
+#     except main.cpp, linking essim_core plus the frontend's own GUI/TUI
+#     toolkit (LIBS);
+#   * the `essim` executable from main.cpp, linking essim_<name> and the shared,
+#     toolkit-free launcher essim_frontend, emitted at the top of the build tree
+#     (./build/essim) whichever frontend produced it.
+#
+# A per-frontend CMakeLists only sets up its toolkit (FetchContent /
+# find_package, and any directory-scoped include dirs / definitions) and then
+# calls this with the toolkit's link targets — no target wiring repeated.
+function(essim_add_frontend name)
+  cmake_parse_arguments(FE "" "" "LIBS" ${ARGN})
+
+  set(dir "${CMAKE_CURRENT_SOURCE_DIR}")
+
+  # Frontend library = every .cpp here except the entry point.
+  file(GLOB FE_SOURCES "${dir}/*.cpp")
+  list(REMOVE_ITEM FE_SOURCES "${dir}/main.cpp")
+
+  add_library(essim_${name} STATIC ${FE_SOURCES})
+  target_link_libraries(essim_${name} PUBLIC essim_core ${FE_LIBS})
+
+  add_executable(essim "${dir}/main.cpp")
+  target_link_libraries(essim PRIVATE essim_${name} essim_frontend)
+  set_target_properties(essim PROPERTIES
+    RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}")
+endfunction()