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>

DESIGN.md

@@ -19,9 +19,10 @@ cmake --build build -j
 - **Core system dependencies** (via `find_package`): `libzip` (`libzip::zip`)
   and `pugixml` (`pugixml::pugixml`) for the ODS importer. Arch:
   `pacman -S libzip pugixml`; Debian/Ubuntu: `libzip-dev libpugixml-dev`.
-- **`libbsdl`** (standalone BSDL parser, LGPL-2.1) — sibling repo at
-  `../libbsdl`, `add_subdirectory` (override `-DBSDL_DIR=...`), linked
-  dynamically **into the core** (`bsdl::bsdl`).
+- **`libbsdl`** (standalone BSDL parser, LGPL-2.1) — linked dynamically **into
+  the core** (`bsdl::bsdl`). Local sibling `../libbsdl` via `add_subdirectory`
+  (override `-DBSDL_DIR=...`) when `${BSDL_DIR}/CMakeLists.txt` exists, otherwise
+  `FetchContent` from `-DBSDL_GIT_REPOSITORY` (`-DBSDL_GIT_TAG`, default `main`).
 - **FTXUI** is fetched by the **tui frontend only**
   (`src/frontends/tui/CMakeLists.txt`), never by the core.
 - Sources are globbed per layer: `src/core/*.cpp` → `essim_core`,