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/frontends/tui/commands_export.cpp

@@ -0,0 +1,70 @@
+#include "frontends/tui/tui.hpp"
+
+#include "core/app/export.hpp"
+#include "core/domain/connect.hpp"
+#include "core/domain/system.hpp"
+
+#include <string>
+#include <vector>
+
+// Thin UI wrapper around app::export_connections — this file only resolves
+// arguments / the file dialog and renders the result. All the actual export
+// (CSV / ODS building, file writing) lives in src/app/export.cpp.
+void Tui::RegisterExportCommands() {
+    commands["export"] = {
+        {{"kind [connections]", Completion::None},
+         {"filename (.csv)",    Completion::Path}},
+        [this](const std::vector<std::string> &args) {
+            if (!sys) { Print("no system: run 'new' first."); return; }
+
+            if (args.empty()) {
+                // Bare → the generic file dialog. The CSV/ODS filter rewrites
+                // the extension; the action below dispatches on it.
+                OpenFileDialog(
+                    "Export — connections",
+                    "export.connections",
+                    "connections.csv",
+                    {{"CSV", ".csv"}, {"ODS", ".ods"}},
+                    [this](const std::string &path) {
+                        Dispatch("export connections " + path);
+                    });
+                return;
+            }
+            if (args.size() != 2) {
+                Print("usage: export <kind> <file>   (or no args for the dialog)");
+                return;
+            }
+            const std::string &kind = args[0];
+            const std::string &path = args[1];
+
+            if (kind != "connections") {
+                ShowError("export: unknown kind '" + kind + "'\n"
+                          "Known kinds: connections");
+                return;
+            }
+
+            app::ExportFormat fmt;
+            if (!app::export_format_from_path(path, fmt)) {
+                ShowError("export: unknown extension — accepted: .csv, .ods");
+                return;
+            }
+
+            app::ExportResult r = app::export_connections(sys.get(), path, fmt);
+            if (!r.ok) {
+                ShowError("export failed:\n" + r.error);
+                return;
+            }
+
+            const bool ods = (fmt == app::ExportFormat::Ods);
+            std::string msg = ods ? "export connections (.ods): "
+                                  : "export connections (.csv): ";
+            if (ods) msg += std::to_string(r.sheets) + " sheet(s), ";
+            Print(msg + std::to_string(r.rows) + " wire(s) → " + path);
+        },
+        /*prompt_for_missing=*/ false,
+        "export structured data to CSV / ODS (kinds: connections; "
+        "bare form opens the file-picker dialog)",
+        /*scriptable=*/ true,
+        /*interactive=*/ true,
+    };
+}