build: split core/ from frontends/; prepare for multiple GUI/TUI targets

Reorganise the tree into business vs frontend as separate directories:
  src/core/{domain,imports,app}   (was system/, imports/, app/)
  src/frontends/tui/              (was tui/ + main.cpp)
  tests/tui/                      (the FTXUI-coupled helper test)
All cross-dir #include paths rewritten; same-dir includes untouched.

CMake: essim_core is the frontend-agnostic business library — links libzip,
pugixml and bsdl, NO GUI toolkit. Each frontend is a self-contained
src/frontends/<name>/ (own CMakeLists, toolkit, main.cpp) that links
essim_core, selected with -DESSIM_FRONTEND=<name> (default tui; 'none' = core +
tests only, no toolkit fetched). FTXUI moved into the tui frontend. Tests are
split: essim_tests links essim_core (no FTXUI), essim_tui_tests links essim_tui.

Verified: default tui build green (ctest 2/2); ESSIM_FRONTEND=none builds the
core + tests with FTXUI never fetched and no `essim` binary.

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

CMakeLists.txt

@@ -11,49 +11,54 @@ project(essim
 
 include(FetchContent)
 
-set(FTXUI_BUILD_DOCS OFF CACHE INTERNAL "")
-set(FTXUI_BUILD_EXAMPLES OFF CACHE INTERNAL "")
-set(FTXUI_BUILD_TESTS OFF CACHE INTERNAL "")
-set(FTXUI_ENABLE_INSTALL OFF CACHE INTERNAL "")
-
-FetchContent_Declare(ftxui
-  GIT_REPOSITORY https://github.com/ArthurSonzogni/FTXUI.git
-  GIT_TAG v6.1.9
-  GIT_SHALLOW TRUE
-)
-FetchContent_MakeAvailable(ftxui)
-
-find_package(libzip REQUIRED)
-find_package(pugixml REQUIRED)
-
-# libbsdl — standalone BSDL parser (LGPL-2.1), dynamically linked from essim
-# (EUPL-1.2, which the LGPL permits). Path overridable via -DBSDL_DIR=...;
-# its CLI and tests are not needed inside essim's build.
+# ----------------------------------------------------------------- core deps
+# libbsdl — standalone BSDL parser (LGPL-2.1), dynamically linked (EUPL-1.2,
+# which the LGPL permits). Override its path with -DBSDL_DIR=...
 set(BSDL_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../libbsdl" CACHE PATH "libbsdl source tree")
 set(BSDL_BUILD_CLI   OFF CACHE BOOL "" FORCE)
 set(BSDL_BUILD_TESTS OFF CACHE BOOL "" FORCE)
 add_subdirectory(${BSDL_DIR} ${CMAKE_BINARY_DIR}/libbsdl)
 
-# Library target = everything except main.cpp; reused by `essim` and `essim_tests`.
-file(GLOB_RECURSE LIB_SOURCES "src/*.cpp")
-list(REMOVE_ITEM LIB_SOURCES "${CMAKE_CURRENT_SOURCE_DIR}/src/main.cpp")
+find_package(libzip REQUIRED)
+find_package(pugixml REQUIRED)
 
-add_library(essim_lib STATIC ${LIB_SOURCES})
-target_include_directories(essim_lib PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/src)
-target_link_libraries(essim_lib
+# =============================================================== essim_core
+# All business logic — domain model, importers, application operations
+# (src/core/{domain,imports,app}). Frontend-agnostic: it links NO GUI/TUI
+# toolkit, so every frontend and the test suite share the exact same core.
+file(GLOB_RECURSE CORE_SOURCES "src/core/*.cpp")
+add_library(essim_core STATIC ${CORE_SOURCES})
+target_include_directories(essim_core PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/src)
+target_link_libraries(essim_core
   PUBLIC
-    ftxui::screen
-    ftxui::dom
-    ftxui::component
     libzip::zip
     pugixml::pugixml
     bsdl::bsdl
 )
 
-add_executable(essim src/main.cpp)
-target_link_libraries(essim PRIVATE essim_lib)
+# =============================================================== frontend(s)
+# Pick the GUI/TUI frontend to build the `essim` binary against. Each frontend
+# is a self-contained src/frontends/<name>/ (own CMakeLists, GUI toolkit, and
+# main.cpp) that links essim_core. "none" builds the core + tests only — no GUI
+# toolkit is fetched. To add a frontend (e.g. a Qt GUI), create
+# src/frontends/gui/ and configure with -DESSIM_FRONTEND=gui.
+set(ESSIM_FRONTEND "tui" CACHE STRING
+    "Frontend to build: a directory name under src/frontends/, or 'none'")
+set_property(CACHE ESSIM_FRONTEND PROPERTY STRINGS tui none)
 
-# Tests
+if(ESSIM_FRONTEND STREQUAL "none")
+  message(STATUS "essim: ESSIM_FRONTEND=none — core + tests only (no frontend, no GUI toolkit)")
+elseif(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/src/frontends/${ESSIM_FRONTEND}/CMakeLists.txt")
+  message(STATUS "essim: building frontend '${ESSIM_FRONTEND}'")
+  add_subdirectory(src/frontends/${ESSIM_FRONTEND})
+else()
+  message(FATAL_ERROR
+    "Unknown ESSIM_FRONTEND '${ESSIM_FRONTEND}' — expected "
+    "src/frontends/${ESSIM_FRONTEND}/CMakeLists.txt, or 'none'.")
+endif()
+
+# =============================================================== tests (core)
+# The suite exercises essim_core only — no frontend, no GUI toolkit.
 include(CTest)
 if(BUILD_TESTING)
   set(CMAKE_POLICY_VERSION_MINIMUM 3.5)
@@ -65,20 +70,35 @@ if(BUILD_TESTING)
   FetchContent_MakeAvailable(doctest)
   unset(CMAKE_POLICY_VERSION_MINIMUM)
 
+  # Core tests — exercise essim_core only (tests/*.cpp, non-recursive, so the
+  # per-frontend tests under tests/<frontend>/ are not pulled in here).
   file(GLOB TEST_SOURCES "tests/*.cpp")
   if(TEST_SOURCES)
     add_executable(essim_tests ${TEST_SOURCES})
-    target_link_libraries(essim_tests PRIVATE essim_lib doctest::doctest)
+    target_link_libraries(essim_tests PRIVATE essim_core doctest::doctest)
     add_test(NAME essim_tests COMMAND essim_tests)
   endif()
+
+  # Per-frontend tests — tests/<frontend>/*.cpp, built and linked against that
+  # frontend's library only when the frontend itself is built.
+  if(TARGET essim_tui)
+    file(GLOB TUI_TEST_SOURCES "tests/tui/*.cpp")
+    if(TUI_TEST_SOURCES)
+      add_executable(essim_tui_tests
+        "${CMAKE_CURRENT_SOURCE_DIR}/tests/doctest_main.cpp" ${TUI_TEST_SOURCES})
+      target_link_libraries(essim_tui_tests PRIVATE essim_tui doctest::doctest)
+      add_test(NAME essim_tui_tests COMMAND essim_tui_tests)
+    endif()
+  endif()
 endif()
 
-# Documentation: Doxygen → XML → custom Python script → doc/api/ (Markdown rendered by gitea).
-# Optional — `doc` target is only created if Doxygen and Python 3 are present.
+# =============================================================== documentation
+# Doxygen → XML → gen_api_md.py → doc/api/, plus `essim --commands-md`. Needs the
+# `essim` binary, so it's only wired when a frontend that provides one is built.
 find_package(Doxygen COMPONENTS doxygen)
 find_package(Python3 COMPONENTS Interpreter)
 
-if(DOXYGEN_FOUND AND Python3_Interpreter_FOUND)
+if(TARGET essim AND DOXYGEN_FOUND AND Python3_Interpreter_FOUND)
   set(DOXYGEN_OUTPUT_DIR "${CMAKE_BINARY_DIR}/doc")
   file(MAKE_DIRECTORY "${DOXYGEN_OUTPUT_DIR}")
   configure_file(
@@ -103,11 +123,10 @@ if(DOXYGEN_FOUND AND Python3_Interpreter_FOUND)
     WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
     COMMENT "Generating documentation (doxygen → gen_api_md.py → doc/api/, essim --commands-md → doc/user/commands.md)"
     VERBATIM)
-elseif(NOT DOXYGEN_FOUND AND NOT Python3_Interpreter_FOUND)
-  message(STATUS "doc: Doxygen and Python 3 not found — `doc` target disabled.")
+elseif(NOT TARGET essim)
+  message(STATUS "doc: no `essim` binary (ESSIM_FRONTEND=none) — `doc` target disabled.")
 elseif(NOT DOXYGEN_FOUND)
-  message(STATUS "doc: Doxygen not found — `doc` target disabled "
-                 "(install via `pacman -S doxygen`).")
+  message(STATUS "doc: Doxygen not found — `doc` target disabled.")
 else()
   message(STATUS "doc: Python 3 interpreter not found — `doc` target disabled.")
 endif()