chore: land janitor round 1 and graft integration

Round 1 findings and fixes, all previously uncommitted:
- FXProcessor per-slot DSP history repair (HEAD does not compile without it)
- SynthVoice::getModulationValue, editor control-lock guard
- ExactCompare.h naming the deliberate exact float comparisons
- KnobLookAndFeel replacing the never-installed SerumLookAndFeel
- 55 first-party compiler warnings to 0
- AGENT.md renamed to AGENTS.md across Source/
- graft repository context graph and its hooks
- JANITOR_TASKS.md and UI_UX_AUDIT.md records

Source/Engine.cpp

@@ -56,16 +56,30 @@ void Engine::reset()
     fx.reset();
     pitchBend = 0.0f;
     modWheel  = 0.0f;
-    activeVoiceCount.store (0, std::memory_order_relaxed);
     mixBuffer.clear();
 }
 
+void Engine::requestReset() noexcept
+{
+    const juce::ScopedLock lock (controlLock);
+    resetRequested = true;
+}
+
 void Engine::captureControls()
 {
     const juce::ScopedTryLock lock (controlLock);
     if (! lock.isLocked())
         return;
 
+    // A preset change asks for a clean start. reset() runs here, on the audio
+    // thread, because the message thread must never touch the voice, filter or
+    // FX state the render is reading.
+    if (resetRequested)
+    {
+        resetRequested = false;
+        reset();
+    }
+
     audioMatrix.connections.assign (matrix.connections.begin(),
         matrix.connections.begin() + juce::jmin (matrix.size(), ModulationMatrix::kMaxConnections));
     for (int i = 0; i < kNumMacros; ++i)
@@ -356,11 +370,6 @@ void Engine::processBlock (juce::AudioBuffer<float>& buffer, juce::MidiBuffer& m
             modWheel = message.getControllerValue() / 127.0f;
     }
     renderUntil (n);
-
-    int active = 0;
-    for (const auto& voice : voices)
-        active += voice.isActive() ? 1 : 0;
-    activeVoiceCount.store (active, std::memory_order_relaxed);
 }
 
 } // namespace serum