build: Digilent backend on by default on UNIX

The driver dlopen's libdjtg/libdmgr and degrades to "no probe" if
they're absent, so building it in has no cost or dependency.
BS_ENABLE_DIGILENT now defaults ON on UNIX (needs <dlfcn.h>); disable
with -DBS_ENABLE_DIGILENT=OFF. Docs updated; also fixes the quartiq
license note in CLAUDE.md (MIT, not BSD-2).

README.md

@@ -26,11 +26,12 @@ in `bsdl_files/`.
 - CMake ≥ 3.10, gcc/clang
 - `readline` (Arch: `readline`, Debian/Ubuntu: `libreadline-dev`)
 - `libftd2xx` for FTDI probes (vendored in `libs/libftd2xx/`)
-- *Optional, for Digilent SMT2/SMT2-NC boards:* the Digilent
+- *To drive a Digilent SMT2/SMT2-NC probe:* the Digilent
   [Adept Runtime](https://digilent.com/shop/software/digilent-adept/)
   installed system-wide (provides `libdjtg.so` + `libdmgr.so`).
   Nothing from Digilent is vendored — the backend is `dlopen`'d at
-  runtime.
+  runtime, so it's built in by default and simply reports no probe if
+  the libs are missing.
 
 ## Build
 
@@ -42,10 +43,10 @@ make
 
 The binary is produced at `build/bs/bs`.
 
-To enable the Digilent SMT2 backend:
+The Digilent SMT2 backend is built by default on Linux. To leave it out:
 
 ```sh
-cmake -DBS_ENABLE_DIGILENT=ON ..
+cmake -DBS_ENABLE_DIGILENT=OFF ..
 ```
 
 ## Run
@@ -114,11 +115,12 @@ Use `help <command>` for per-command help.
   in `modules/config/config.script` for pin mapping and TCK frequency.
 - **SEGGER J-Link**
 - **Linux GPIO** (sysfs; deprecated on recent kernels, libgpiod migration TBD)
-- **Digilent JTAG-SMT2 / SMT2-NC** — optional, built when
-  `-DBS_ENABLE_DIGILENT=ON`. Required for the USB-JTAG on Xilinx eval
-  boards like the KCU105: those modules ship a Digilent-proprietary
-  firmware that does not respond to plain MPSSE, so the FTDI driver
-  appears to enumerate them but the JTAG chain stays silent.
+- **Digilent JTAG-SMT2 / SMT2-NC** — built in by default on Linux
+  (`-DBS_ENABLE_DIGILENT=OFF` to drop it). Required for the USB-JTAG on
+  Xilinx eval boards like the KCU105: those modules ship a
+  Digilent-proprietary firmware that does not respond to plain MPSSE,
+  so the FTDI driver appears to enumerate them but the JTAG chain stays
+  silent.
 
 ## Known Xilinx caveats