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/app/export.hpp

@@ -0,0 +1,34 @@
+#ifndef _APP_EXPORT_HPP_
+#define _APP_EXPORT_HPP_
+
+#include <string>
+
+class System;
+
+// Application layer: UI-independent operations that any frontend (TUI, GUI, …)
+// can call. No console, no dialogs, no FTXUI — just System in, result out.
+namespace app {
+
+enum class ExportFormat { Csv, Ods };
+
+// Outcome of an export. The only side effect is writing the target file; the
+// caller renders `error` / the stats however it likes.
+struct ExportResult {
+    bool ok = false;
+    std::string error;  ///< human-readable, set when !ok
+    int sheets = 0;     ///< ODS: number of sheets (one per connection); 0 for CSV
+    int rows = 0;       ///< wires written
+};
+
+// Map a filename extension (.csv / .ods, case-insensitive) to a format.
+// Returns false if the extension is neither.
+bool export_format_from_path(const std::string &path, ExportFormat &out);
+
+// Export the system's connections to `path` in `format`. Builds the file and
+// returns stats or an error. Pure core — safe to call from any frontend.
+ExportResult export_connections(const System *sys, const std::string &path,
+                                ExportFormat format);
+
+}  // namespace app
+
+#endif  // _APP_EXPORT_HPP_