JUCE SDK integration.

docs/ROADMAP.md

@@ -3440,6 +3440,64 @@ configuration correctly rejects the unsupported platform; no Windows build or
 ASIO hardware result is claimed.
 
 
+---
+
+## Phase 107 — JUCE Audio, Plugin, DSP and Interface Examples
+
+### Goal
+
+Use JUCE's relevant audio framework capabilities with the actual xtc C99 engine:
+AudioProcessor/plugin wrappers, standalone audio/MIDI devices, a native editor,
+parameter/state management, DSP effects, MIDI files, WAV export and VST3 hosting.
+Keep this integration optional and externally supplied. This user-requested
+phase can proceed alongside the unfinished platform work in Phase 106; it does
+not replace the main FLTK application or move the C engine into C++.
+
+### Tasks and acceptance criteria
+
+- [x] Add an offline-by-default external JUCE build and explain its
+      AGPLv3/commercial licensing, dependencies and packaging boundary.
+- [x] Build working VST3 and standalone synth targets using xtc Soft Keys through
+      the public C API; negotiate mono/stereo and handle variable host blocks.
+- [x] Provide a native JUCE editor with attached host parameters, on-screen MIDI
+      keyboard, peak display, panic and transactional versioned preset files.
+- [x] Use JUCE DSP for a low-pass filter, reverb and smoothed output gain, with
+      all processing storage prepared outside processBlock.
+- [x] Implement bounded MIDI queues/overflow panic and preserve host MIDI sample
+      offsets without callback allocation, I/O, GUI calls or host-side locks.
+- [x] Provide a console example for Standard MIDI File import with tempo
+      conversion and float32 stereo WAV rendering with atomic no-clobber output.
+- [x] Demonstrate explicit JUCE VST3 discovery, parameter inspection, state
+      capture/restore and actual MIDI processing against the built example.
+- [x] Test C-render equivalence, stereo/mono, variable blocks, reset, invalid
+      preparation, queue overflow, preset recovery, editor rendering/reopening
+      and loading the JUCE VST3 from xtc's existing C host.
+- [x] Build with warnings enabled, run the focused integration checks and retain
+      ordinary C/FLTK builds/tests without a JUCE dependency.
+
+### Implementation status
+
+Implemented and qualified on Linux with external JUCE 9.0.4 revision
+`3fb5cbc0340758596dac39f7c8de47d562b4896e`. See [JUCE.md](JUCE.md) and
+[examples/juce](../examples/juce/README.md) for reproduction.
+
+Validation (2026-11-06):
+
+- `make -j4 all test`: ordinary C/FLTK build and regression suite passed.
+- `make JUCE_ROOT=/path/to/JUCE test-juce`: warning-enabled VST3, standalone
+  and CLI builds; processor/JUCE-host CTests; file/editor acceptance checks;
+  and the existing C-host integration test passed under Xvfb.
+- Steinberg VST 3.8.1 validator: 47 tests passed, zero failed.
+- Rendered the demo to a 216000-frame 48 kHz stereo float32 WAV; inspected
+  the actual 1560 × 980 editor snapshot. No physical audio device was opened.
+- C-host all-off uses individual note-offs, so acceptance checks finite release
+  and eventual silence; the JUCE host also verifies immediate MIDI all-off timing.
+
+ Broader adoption in
+the main DAW, additional plugin formats, Windows/macOS portability, native ASIO
+and physical-device latency/loopback qualification remain separate work. Merely
+building JUCE's standalone wrapper does not establish hardware acceptance.
+
 ---
 
 ## Recommended Execution Order