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>

data/README.md

@@ -0,0 +1,18 @@
+# Sample BSDL files
+
+This directory is intentionally **not** populated with a BSDL database.
+
+BSDL files are mandated to be freely available by IEEE 1149.1, but their
+redistribution terms vary per vendor, so they are not vendored here. Point the
+tests and tools at a local directory instead.
+
+Sources:
+
+- Vendor sites (authoritative): TI, AMD/Xilinx, Intel/Altera, Lattice, NXP, ST…
+- [bsdl.info](https://bsdl.info/) — community aggregate (~14k files).
+- Quartus / Vivado installs bundle BSDL for their FPGAs.
+- For quick local testing, the sibling `bs_explorer` repo ships a few under
+  `bs_explorer/data/bsdl_files/` (xcku15p, xcku040, m2gl010t).
+
+> Note: a *configured* FPGA's BSDL differs from the blank device — use the
+> blank-device BSDL for connectivity checks.