libbsdl: scaffold + working BSDL parser (struct + JSON, C ABI)

Standalone LGPL-2.1 parser for BSDL (IEEE 1149.1), shared by essim and
bs_explorer. The parser is ported from the Viveris JTAG Core loader,
decoupled from jtag_core, and extended with two extractions the original
lacks:
  - PIN_MAP_STRING -> per-port physical package pin, scalar and vector;
  - TAP_SCAN_*     -> TAP signal roles (TDI/TDO/TMS/TCK/TRST).

Exposes a stable C ABI (bsdl_parse_file/buffer -> bsdl_t, bsdl_to_json)
with a dependency-free JSON serializer and a bsdl2json CLI. CMake builds a
versioned shared library with install/export rules for find_package(bsdl).

Verified against m2gl010t, xcku040 and xcku15p (100% pin mapping, correct
IDCODEs and TAP roles); api and parse regression tests pass, clean build.

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

.gitignore

@@ -0,0 +1,26 @@
+# build trees
+/build/
+/build-*/
+/out/
+cmake-build-*/
+
+# CMake debris (if ever configured in-source)
+CMakeCache.txt
+CMakeFiles/
+cmake_install.cmake
+CTestTestfile.cmake
+install_manifest.txt
+*.cmake.user
+
+# editors
+.vscode/
+.idea/
+*.swp
+
+# artifacts
+*.o
+*.a
+*.so
+*.so.*
+*.dll
+*.dylib