digilent: add driver skeleton + CMake option BS_ENABLE_DIGILENT

Stub Digilent JTAG-SMT backend, off by default. Wiring only: option,
conditional sources, dl link, drivers_list registration. Detect()
returns 0 for now; dlopen + real implementation in follow-up commits.

CMakeLists.txt

@@ -2,6 +2,11 @@ cmake_minimum_required(VERSION 3.10)
 
 project(BoundaryScanExplorer)
 
+# Optional backends. Off by default: no proprietary runtime dependency.
+option(BS_ENABLE_DIGILENT
+       "Enable Digilent JTAG-SMT* backend (dlopen libdjtg.so / libdmgr.so at runtime; requires Adept Runtime installed)"
+       OFF)
+
 # script and jtag_core must be the last linked archive for the application to compile
 set(BS_MODULES script jtag_core)
 set(DIR_MODULES ${CMAKE_SOURCE_DIR}/modules)