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/GUI/LFODisplay.h

@@ -3,12 +3,17 @@
 #include <JuceHeader.h>
 #include "../Resources.h"
 #include "../Params.h"
+#include "../LFO.h"
 
 namespace serum
 {
 
 // ===========================================================================
 // LFO shape view. Step-sequencer and freehand shapes are click/drag editable.
+// The preview evaluates the same lfoShapeValue() function as the audio path, so
+// the drawn shape cannot drift from the sound. Sample and hold has no phase
+// function (the audio level is stateful), so its preview is a labelled
+// schematic rather than a fabricated sequence.
 // ===========================================================================
 class LFODisplay : public juce::Component
 {
@@ -21,13 +26,21 @@ public:
     void mouseDown (const juce::MouseEvent& e) override;
     void mouseDrag (const juce::MouseEvent& e) override;
 
+    // Level the sample-and-hold preview draws. The audio LFO holds a level that
+    // the display cannot observe on the message thread, so the preview shows a
+    // flat schematic line at this value instead of inventing a sequence.
+    static constexpr float kSampleHoldLevel = 0.0f;
+
+    // Value of the current shape at `phase` (0..1); the audio path calls the
+    // same lfoShapeValue() with its live sample-and-hold level.
+    float valueAt (float phase) const noexcept;
+
 private:
     int   shape = 0;
     std::vector<float> shapeData;
     int   steps = 16;
     std::function<void (const std::vector<float>&, int)> onEdited;
 
-    float valueAt (float phase) const noexcept;
     void  editAt (float x, float y);
 };