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>

src/bsdl_strings.h

@@ -0,0 +1,26 @@
+/*
+ * libbsdl - internal keyword lookup tables.
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * These map BSDL tokens onto the public enums. They are the seed for the
+ * parser port from Viveris, and already extend it with the two attributes
+ * Viveris omits: LINKAGE ports and the TAP_SCAN_* roles.
+ */
+#ifndef LIBBSDL_BSDL_STRINGS_H
+#define LIBBSDL_BSDL_STRINGS_H
+
+typedef struct bsdl_kw {
+    const char* name;  /* BSDL token, upper-cased before lookup. */
+    int         code;  /* corresponding bsdl_* enum value. */
+} bsdl_kw_t;
+
+extern const bsdl_kw_t bsdl_dir_kw[];      /* -> bsdl_dir_t      */
+extern const bsdl_kw_t bsdl_cell_kw[];     /* -> bsdl_cell_t     */
+extern const bsdl_kw_t bsdl_bittype_kw[];  /* -> bsdl_bittype_t  */
+extern const bsdl_kw_t bsdl_state_kw[];    /* -> bsdl_state_t    */
+extern const bsdl_kw_t bsdl_tap_kw[];      /* -> bsdl_tap_role_t */
+
+/* Linear lookup; returns the first table entry's code (UNKNOWN/NONE) if absent. */
+int bsdl_kw_lookup(const bsdl_kw_t* table, const char* name);
+
+#endif /* LIBBSDL_BSDL_STRINGS_H */