Added an alternative GUI based on JUCE; I like FLTK more...

docs/BUILD.md

@@ -79,6 +79,7 @@ lists, dependencies, SDK and packaging rules in CMake.
 | Cache option | Default | Meaning |
 | --- | --- | --- |
 | `BUILD_TESTING` | `ON` | Build/register the test suite |
+| `XTC_BUILD_JUCE_GUI` | `OFF` | Build the JUCE workstation frontend (`xtc-juce`), independently of FLTK; requires `XTC_JUCE_ROOT` |
 | `XTC_BUILD_GUI` | `ON` | Discover FLTK and build the application when available |
 | `XTC_WITH_JACK` | `ON` | Discover/build the JACK/PipeWire backend |
 | `XTC_WITH_LADSPA` | `ON` | Enable LADSPA when the system header exists |
@@ -191,3 +192,21 @@ Clean products with `cmake --build build --target clean`; to discard the cache,
 remove only the selected build directory. Never configure in the source tree.
 Historical validation entries in ROADMAP describe the build used at that time;
 Phase 108 records the CMake migration and its verified results.
+
+## Optional JUCE workstation frontend
+
+See [src/gui/JUCE/README.md](../src/gui/JUCE/README.md) for the component map and
+licence requirements. The SDK is supplied by the user; no download occurs.
+
+```sh
+cmake -S . -B build/juce-gui -DCMAKE_BUILD_TYPE=Debug \
+  -DXTC_BUILD_GUI=OFF -DXTC_BUILD_JUCE_GUI=ON -DXTC_JUCE_ROOT=/path/to/JUCE
+cmake --build build/juce-gui --parallel 4 --target xtc-juce test_juce_gui
+./build/juce-gui/xtc-juce examples/projects/playback-test.xtc
+ctest --test-dir build/juce-gui -R juce-gui --output-on-failure -j1
+```
+
+`XTC_BUILD_GUI=ON` builds both frontends. `XTC_BUILD_JUCE` continues to mean
+JUCE plugin/processor examples and can be enabled independently or alongside
+the workstation frontend. A missing SDK is a configuration error only when a
+JUCE option is enabled. The existing default FLTK build is unchanged.