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/tui_helpers.hpp

@@ -0,0 +1,39 @@
+#ifndef _TUI_HELPERS_HPP_
+#define _TUI_HELPERS_HPP_
+
+#include <ftxui/dom/elements.hpp>
+
+#include <string>
+#include <vector>
+
+// Free helpers shared across the TUI translation units.
+
+// Highlight the label of a focused field in interactive screens. Used so the
+// user can see at a glance which field will receive their next keystroke.
+inline ftxui::Element FocusLabel(ftxui::Element e, bool focused) {
+    return focused ? (e | ftxui::inverted) : e;
+}
+
+std::string ToLower(std::string s);
+
+// ---- Context help panel (right-column on every screen) ----
+struct HelpEntry {
+    std::string key;     ///< Key or chord label ("Tab", "Enter", "Ctrl-P", "s").
+    std::string desc;    ///< Short description of what it does.
+};
+
+// Renders a vertical help column: bold title, separator, then a two-column
+// list of (key, desc). Fixed width so the layout is consistent.
+ftxui::Element RenderHelpPanel(const std::string &title,
+                                const std::vector<HelpEntry> &entries);
+
+// Case-insensitive natural-order comparison: digit runs compared as integers,
+// letters compared after std::tolower.
+bool NaturalLess(const std::string &a, const std::string &b);
+
+std::string LongestCommonPrefix(const std::vector<std::string> &v);
+
+// Whitespace tokeniser with `"…"` quoting (preserved-as-content).
+std::vector<std::string> Tokenize(const std::string &s);
+
+#endif // _TUI_HELPERS_HPP_