docs: rewrite DESIGN + README for the core/frontends structure

DESIGN.md: new layered Build section (essim_core + ESSIM_FRONTEND, FTXUI per
frontend, split tests), a rewritten Layout tree (src/core/{domain,imports,app},
src/frontends/tui, tests/{,tui}), and a new "Architecture — core vs frontends"
section stating the rule (core never depends on a frontend) with the export
operation as the worked example. README: layered-build note, FTXUI-is-the-tui-
frontend's dependency, core-vs-frontend test split, nested project-layout tree.

Also keep the binary at ./build/essim via RUNTIME_OUTPUT_DIRECTORY now that the
exe is produced from the frontend subdir.

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

README.md

@@ -15,6 +15,13 @@ cmake --build build -j
 ./build/essim
 ```
 
+The build is **layered**: `essim_core` is the frontend-agnostic business
+library (domain + importers + operations); the `essim` binary comes from a
+**frontend** under `src/frontends/<name>/` that links it. Select one with
+`-DESSIM_FRONTEND=<name>` (default `tui`); `-DESSIM_FRONTEND=none` builds the
+core + tests only, with no GUI toolkit fetched. Architecture in
+[`DESIGN.md`](DESIGN.md).
+
 Inside the shell, type `help` for the live command list — or read the
 auto-generated reference at [`doc/user/commands.md`](doc/user/commands.md).
 A worked bring-up script is at [`test/system.essim`](test/system.essim);
@@ -41,10 +48,14 @@ Step-by-step walkthroughs for both the batch and TUI workflows are in
   `../libbsdl`, pulled in via `add_subdirectory` and linked dynamically.
   Override its location with `-DBSDL_DIR=/path/to/libbsdl`. Powers the
   `attach-bsdl` command and the pin/JTAG checks.
-- Fetched automatically at configure time via `FetchContent` (nothing to
-  install): **FTXUI** v6.1.9 and **doctest** v2.4.11.
+- Fetched automatically via `FetchContent` (nothing to install): **FTXUI**
+  v6.1.9 — only when building the **tui** frontend — and **doctest** v2.4.11
+  for the tests.
 - Optional, only for the `doc` target: **doxygen** and **python3**.
 
+libzip, pugixml and libbsdl are the **core** dependencies; FTXUI belongs to the
+tui frontend, so a `-DESSIM_FRONTEND=none` build needs none of it.
+
 ## Tests
 
 ```sh
@@ -53,6 +64,9 @@ Step-by-step walkthroughs for both the batch and TUI workflows are in
 ctest --test-dir build
 ```
 
+`ctest` runs `essim_tests` (core — links `essim_core`, no GUI toolkit) and
+`essim_tui_tests` (the FTXUI frontend's tests, under `tests/tui/`).
+
 Skip building tests entirely:
 
 ```sh
@@ -76,12 +90,17 @@ cmake --build build --target doc      # needs doxygen + python3
 ## Project layout
 
 ```
-src/system/   domain model (Module/Part/Pin/Signal, Connection, Transform, …)
-src/imports/  Mentor / Altium / ODS netlist importers
-src/tui/      FTXUI shell (commands, screens, completion, history)
-tests/        doctest suite
-doc/          api/ + user/ Markdown trees, Doxyfile.in, gen_api_md.py
-test/         sample netlists + system.essim bring-up script
+src/
+  core/                 business logic, NO GUI toolkit (→ libessim_core)
+    domain/             model (Module/Part/Pin/Signal, Connection, Transform…) + analyses
+    imports/            Mentor / Altium / ODS netlist importers + ODS writer
+    app/                use-case operations (export → CSV/ODS, …)
+  frontends/            one dir per GUI/TUI engine, each links essim_core
+    tui/                FTXUI shell + main.cpp (→ libessim_tui + the `essim` binary)
+tests/                  core tests (link essim_core)
+  tui/                  frontend tests (link essim_tui)
+doc/                    api/ + user/ Markdown, Doxyfile.in, gen_api_md.py
+test/                   sample netlists + system.essim bring-up script
 ```
 
 Full layout & rationale in [`DESIGN.md`](DESIGN.md).