build: fetch libbsdl from git when ../libbsdl is absent

Fall back to FetchContent (BSDL_GIT_REPOSITORY/BSDL_GIT_TAG) so a fresh
clone configures without a manual side-checkout.

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

README.md

@@ -44,10 +44,11 @@ Step-by-step walkthroughs for both the batch and TUI workflows are in
   - Debian/Ubuntu — `sudo apt install libzip-dev libpugixml-dev`
   - Arch — `sudo pacman -S libzip pugixml`
   - Fedora — `sudo dnf install libzip-devel pugixml-devel`
-- **libbsdl** — the standalone BSDL parser, a sibling repo expected at
-  `../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.
+- **libbsdl** — the standalone BSDL parser, linked dynamically. Uses a sibling
+  checkout at `../libbsdl` (override `-DBSDL_DIR=/path/to/libbsdl`) if present,
+  else fetched from git automatically (`-DBSDL_GIT_REPOSITORY=...`,
+  `-DBSDL_GIT_TAG=...`, default branch `main`). Powers the `attach-bsdl` command
+  and the pin/JTAG checks.
 - 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.