Interactive net screen, main + per-screen title bars, focus highlight, help split.

- New `net` full-screen layout (`screen_net.cpp`, `screen_idx = 5`): three
  columns (module menu / signal filter + menu / live BFS result). Bare
  `net` opens the screen; `net <m> <s>` keeps the inline path.
- Main screen grows a title bar: " essim — system digital twin "
  (bold + dim) on the left, live "N module(s), M connection(s)" on
  the right.
- Every interactive screen now renders the same breadcrumb at the top:
  " essim → <name>  — <short description> ", followed by a separator.
- `tui_helpers.hpp` exports `FocusLabel(elem, focused)`. Every
  interactive screen wraps its field labels with it so the active
  field's label flips to inverted video. Buttons (Connect, Apply)
  invert as a whole.
- `CommandSpec` gains a `bool interactive`. `help` (no args) splits
  the listing into "Interactive (open a full-screen mode)" and
  "Other". `help <name>` tags interactive entries with [interactive]
  and explains the bare-vs-inline duality.
- `DESIGN.md` (renamed from `CLAUDE.md`): refreshed Layout, TUI, and
  screen-recipe sections to cover the new field, the title idiom,
  FocusLabel, the `net` screen, and the event-paced `Computing…`
  modal during `source`.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/tui/tui.hpp

@@ -33,6 +33,7 @@ class Tui {
         bool prompt_for_missing = true;
         std::string description;
         bool scriptable = true;
+        bool interactive = false;  ///< opens a full-screen mode when called bare
     };
 
     // ---- Shell state ----
@@ -99,6 +100,14 @@ class Tui {
     bool loading_prev_in_source;
     ftxui::ScreenInteractive *screen_ptr;  ///< set in Run() so Source() can post events.
 
+    // ---- Net screen state ----
+    std::vector<std::string> net_modules;
+    int net_module_idx;
+    std::string net_sig_filter;
+    std::vector<std::string> net_sigs;     ///< rebuilt every frame from filter
+    int net_sig_idx;
+    int net_focus_idx;
+
     // ---- Set-type screen state ----
     std::vector<std::string> settype_modules;
     int settype_m_idx;
@@ -152,6 +161,7 @@ private:
     ftxui::Component BuildConnectScreen();
     ftxui::Component BuildSettypeScreen();
     ftxui::Component BuildExploreScreen();
+    ftxui::Component BuildNetScreen();
 };
 
 #endif // _TUI_HPP_