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

docs/ROADMAP.md

@@ -3739,6 +3739,47 @@ Play/Stop/Space/stereo check also passed. See the integration-test guide for com
 
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+## Phase 113 — Optional JUCE workstation frontend
+
+### Tasks and acceptance criteria
+
+- [x] Preserve the FLTK sources and default build; add an independently optional
+      `XTC_BUILD_JUCE_GUI` workstation under `src/gui/JUCE`.
+- [x] Require a user-supplied SDK through `XTC_JUCE_ROOT`, with an explicit error
+      when enabled without the SDK and no SDK discovery/download in normal builds.
+- [x] Port the main window, sequencer/master and child views, graph, mixer,
+      automation, piano roll, velocity editing, sample editor, plugin dialogs,
+      insert chains, preferences, searchable commands and support widgets.
+- [x] Use JUCE components, controls, graphics, native events, dialogs and timers;
+      keep project/history/DSP/backend/plugin APIs in C and preserve `.xtc` files.
+- [x] Allow JUCE-only, FLTK-only, both and core-only configurations; share the
+      externally supplied SDK with the independently optional plugin examples.
+- [x] Include the JUCE binary in the owned install/uninstall inventory when selected.
+- [x] Pass the retained GUI regression workflows against JUCE, including failure
+      rollback, child ownership, gestures, preferences and MIDI device fallback.
+- [x] Verify native JUCE transport events and finite stereo playback with the
+      bundled test song and missing saved MIDI devices.
+- [x] Validate the original FLTK build/tests and document final build results.
+
+### Implementation notes
+
+The frontend retains the original C-facing command structure and filenames.
+`ui.*` maps the retained coordinates and callback conventions to JUCE, with no
+FLTK headers or runtime dependency. Native controls may look different. Both
+frontends continue to use the existing JACK/PipeWire backend. See the
+[frontend source and build guide](../src/gui/JUCE/README.md).
+JUCE is an optional external AGPLv3/commercial SDK; normal builds do not link it.
+Qualification is for Linux, rather than a new cross-platform audio backend.
+The original 57 FLTK/core tests, core-only build, missing-SDK diagnostic, shared
+SDK configuration and staged install/uninstall checks passed. The frontend was
+built with warnings enabled against external JUCE 8.0.15. Native Play/Stop/Space,
+expanded track rendering and finite stereo playback passed with missing saved
+MIDI devices. All 12 JUCE GUI workflows passed, with focused reruns after the
+native focus/blur adjustments. The [workstation capture](../screenshots/juce-workstation.png)
+comes from this real-backend regression, with no physical routes changed.
+
+---
+
 ## Recommended Execution Order
 
 Reorder the phases when the audit reveals a blocking dependency. Do not mark a