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/imports/ods_writer.hpp

@@ -0,0 +1,62 @@
+#ifndef _IMPORTS_ODS_WRITER_HPP_
+#define _IMPORTS_ODS_WRITER_HPP_
+
+#include <string>
+#include <vector>
+
+// Minimal OpenDocument Spreadsheet (.ods) writer. Backed by libzip +
+// pugixml (already in the build for the ODS *importer*). Produces
+// multi-sheet workbooks; each sheet is a 2-D grid of string cells.
+//
+// Usage:
+//     OdsWriter w;
+//     auto *s1 = w.add_sheet("connection A");
+//     s1->set(0, 0, "header1");
+//     s1->set(0, 1, "header2");
+//     s1->set(1, 0, "value");
+//     std::string err;
+//     bool ok = w.save("out.ods", err);
+//
+// Limitations (intentional, the format is huge):
+//   - string cells only (no numbers / dates / formulas)
+//   - no styles, no merged cells, no formatting
+//   - a single empty row is emitted when a row has at least one set cell;
+//     trailing empty rows are skipped.
+
+class OdsSheet {
+public:
+    explicit OdsSheet(std::string name) : name_(std::move(name)) {}
+
+    void set(int row, int col, std::string value);
+
+    // Index of the row that holds the column headers — gets the bold/grey
+    // style, anchors the freeze, and is the first row of the auto-filter
+    // range. Rows above it are rendered un-styled (a place for free-form
+    // metadata such as the connection name). Default 0 = first row is
+    // the header (no meta block).
+    void set_header_row(int r) { header_row_ = r; }
+    int  header_row() const    { return header_row_; }
+
+    const std::string &name() const { return name_; }
+    int rows() const { return (int)cells_.size(); }
+    int cols() const;
+    const std::string &cell(int row, int col) const;
+
+private:
+    std::string name_;
+    int header_row_ = 0;
+    // Row-major sparse storage: cells_[r][c] = value. Rows/cols are grown
+    // lazily on set().
+    std::vector<std::vector<std::string>> cells_;
+};
+
+class OdsWriter {
+public:
+    OdsSheet *add_sheet(const std::string &name);
+    bool save(const std::string &path, std::string &error) const;
+
+private:
+    std::vector<OdsSheet> sheets_;
+};
+
+#endif  // _IMPORTS_ODS_WRITER_HPP_