docs: document the PinSpec refactor + add a Dependencies section

DESIGN.md: Pin now carries a PinSpec (function/direction/pad/source);
expected_signal_type() is a derived accessor; pin_role() returns a PinSpec.
README.md: dedicated Dependencies section with libzip/pugixml install
commands for Debian/Ubuntu, Arch and Fedora.

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

README.md

@@ -15,19 +15,23 @@ cmake --build build -j
 ./build/essim
 ```
 
-Requirements:
-
-- A C++17 compiler.
-- CMake 3.14+.
-- System packages: `libzip` and `pugixml` (Arch: `pacman -S libzip pugixml`).
-- FTXUI (v6.1.9) and doctest (v2.4.11) are fetched at configure time via
-  `FetchContent`.
-
 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);
 load it with `source test/system.essim`.
 
+## Dependencies
+
+- **C++17 compiler** and **CMake 3.14+**.
+- System libraries **libzip** and **pugixml** — install the *development*
+  packages:
+  - Debian/Ubuntu — `sudo apt install libzip-dev libpugixml-dev`
+  - Arch — `sudo pacman -S libzip pugixml`
+  - Fedora — `sudo dnf install libzip-devel pugixml-devel`
+- Fetched automatically at configure time via `FetchContent` (nothing to
+  install): **FTXUI** v6.1.9 and **doctest** v2.4.11.
+- Optional, only for the `doc` target: **doxygen** and **python3**.
+
 ## Tests
 
 ```sh