docs: DESIGN — Frontend interface + frontend_main, app/ ops, layout

Reflect the new shared frontends layer (frontend.hpp / frontend_main) in the
Architecture section and layout tree, and list the verify/connect/load app ops
alongside export.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

DESIGN.md

@@ -61,10 +61,15 @@ src/
       import_base.hpp / import_{mentor,altium,ods}.{hpp,cpp} / ods_writer.{hpp,cpp}
     app/                 -- application operations (UI-independent use cases)
       export.{hpp,cpp}     export_connections(System*, path, format) -> ExportResult
+      verify.{hpp,cpp}     verify(System*) -> VerifyReport (the 7 verify passes)
+      connect.{hpp,cpp}    connect_parts(System*, m1,p1, m2,p2) -> ConnectResult
+      load.{hpp,cpp}       load_module(System*, name, path, ImportType) -> LoadResult
   frontends/             -- one directory per GUI/TUI engine; each links essim_core
+    frontend.hpp         -- abstract Frontend interface (BootDispatch/Dump*/Run)
+    frontend_main.{hpp,cpp} -- frontend_main(argc,argv,Frontend&): argv + boot/batch/run
     tui/                 -- FTXUI shell (builds libessim_tui + the `essim` binary)
       CMakeLists.txt       fetches FTXUI; builds essim_tui + essim
-      main.cpp             entry point (CLI flags -> Tui)
+      main.cpp             entry point: construct Tui, call frontend_main
       tui.{hpp,cpp}        class Tui (state + Run() + screen-mode event dispatch)
       tui_helpers.{hpp,cpp}  ToLower, NaturalLess, RenderHelpPanel
       shell.cpp            Print, Submit, Dispatch, Source / ProcessNextSourceLine
@@ -93,8 +98,15 @@ The hard rule: **`src/core/` never depends on a frontend** — no `#include
   inside its lambda. The TUI command is now a thin wrapper — resolve args/dialog
   → call the core op → render the result.)
 - **Frontends** (`frontends/<name>/`) — thin: map UI events to core calls and
-  render results. Add one by creating `src/frontends/<name>/CMakeLists.txt` (build
-  `essim_<name>` linking `essim_core`, produce the `essim` binary) and configuring
+  render results. Each implements the **`Frontend`** interface
+  (`frontends/frontend.hpp`: `BootDispatch`, `DumpCommandsMd`, `DumpOutput`,
+  `Run`). The process entry is shared and frontend-agnostic:
+  `frontend_main(argc, argv, Frontend&)` (`frontends/frontend_main.cpp`, built
+  into the toolkit-free `essim_frontend` lib) parses the CLI flags and drives the
+  boot → batch/run flow through the interface; a frontend's `main()` is just
+  *construct the concrete Frontend, call `frontend_main`*. Add one by creating
+  `src/frontends/<name>/CMakeLists.txt` (build `essim_<name>` linking `essim_core`,
+  produce the `essim` binary linking `essim_frontend`) and configuring
   `-DESSIM_FRONTEND=<name>`.
 
 Because the core links no toolkit, the suite links `essim_core` directly and