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>

src/core/domain/system.hpp

@@ -0,0 +1,30 @@
+#ifndef _SYSTEM_HPP_
+#define _SYSTEM_HPP_
+
+#include "core/imports/import_base.hpp"
+
+#pragma once
+class Modules; ///< Forward declaration of the Modules class.
+class Connections; ///< Forward declaration of the Connections class.
+
+
+enum class ImportType {
+    IMPORT_MENTOR,
+    IMPORT_ALTIUM,
+    IMPORT_ODS,
+};
+
+class System
+{
+    Modules *mods;
+    Connections *conns;
+
+public:
+    System();
+    void Load(std::string module_name, std::string filename, ImportType type);
+    Modules *modules() const { return mods; }
+    Connections *connections() const { return conns; }
+    ~System();
+};
+
+#endif // _SYSTEM_HPP_