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.cpp

@@ -1,52 +1,28 @@
 #include "LFODisplay.h"
+#include "../ExactCompare.h"
 
 namespace serum
 {
 
 void LFODisplay::setShapeData (const std::vector<float>& data, int s)
 {
-    const int newSteps = juce::jlimit (2, 64, s);
-    const size_t dataSize = std::min (data.size(), size_t (64));
-    if (steps == newSteps && shapeData.size() == 64
+    const int newSteps = juce::jlimit (2, LFO::kShapePoints, s);
+    const auto dataSize = (std::ptrdiff_t) std::min (data.size(), size_t (LFO::kShapePoints));
+    if (steps == newSteps && (int) shapeData.size() == LFO::kShapePoints
         && std::equal (data.begin(), data.begin() + dataSize, shapeData.begin())
-        && std::all_of (shapeData.begin() + dataSize, shapeData.end(), [] (float v) { return v == 0.0f; }))
+        && std::all_of (shapeData.begin() + dataSize, shapeData.end(), [] (float v) { return exactly (v, 0.0f); }))
         return;
 
     steps = newSteps;
     shapeData.assign (data.begin(), data.begin() + dataSize);
-    shapeData.resize (64, 0.0f);
+    shapeData.resize ((size_t) LFO::kShapePoints, 0.0f);
     repaint();
 }
 
 float LFODisplay::valueAt (float phase) const noexcept
 {
-    switch ((LfoShape) shape)
-    {
-        case LfoShape::Sine:      return std::sin (phase * 6.2831853f);
-        case LfoShape::Triangle:  return 1.0f - 4.0f * std::abs (phase - 0.5f);
-        case LfoShape::Saw:       return 2.0f * phase - 1.0f;
-        case LfoShape::Square:    return (phase < 0.5f) ? 1.0f : -1.0f;
-        case LfoShape::SampleHold:
-        {
-            const int step = juce::jlimit (0, 15, (int) (phase * 16.0f));
-            return ((step * 2654435761u) & 0xffffu) / 32768.0f - 1.0f;
-        }
-        case LfoShape::StepSeq:
-        {
-            const int idx = juce::jlimit (0, steps - 1, (int) (phase * steps));
-            return shapeData.empty() ? 0.0f : shapeData[(size_t) idx];
-        }
-        case LfoShape::Freehand:
-        {
-            if (shapeData.empty()) return 0.0f;
-            const float pos = phase * (float) (steps - 1);
-            const int i0 = (int) pos;
-            const int i1 = juce::jmin (i0 + 1, steps - 1);
-            const float frac = pos - (float) i0;
-            return shapeData[(size_t) i0] * (1.0f - frac) + shapeData[(size_t) i1] * frac;
-        }
-        default: return 0.0f;
-    }
+    return lfoShapeValue (phase, (LfoShape) shape, steps,
+                          shapeData.data(), (int) shapeData.size(), kSampleHoldLevel);
 }
 
 void LFODisplay::paint (juce::Graphics& g)
@@ -75,6 +51,26 @@ void LFODisplay::paint (juce::Graphics& g)
         }
     }
 
+    // Sample and hold is stateful in the audio path, so the preview draws the
+    // held level as a flat schematic with its hold steps marked, and labels it,
+    // rather than inventing a sequence the synth will never play.
+    if (shape == (int) LfoShape::SampleHold)
+    {
+        const float stepW = (b.getWidth() - 8.0f) / (float) steps;
+        g.setColour (theme::outline);
+        for (int i = 1; i < steps; ++i)
+        {
+            const float x = b.getX() + 4.0f + stepW * (float) i;
+            g.drawLine (x, midY - b.getHeight() * 0.28f, x, midY + b.getHeight() * 0.28f, 1.0f);
+        }
+
+        g.setColour (theme::textDim);
+        g.setFont (10.0f);
+        g.drawText ("S&H schematic",
+                    juce::Rectangle<float> (b.getX(), b.getY(), b.getWidth(), 14.0f).toNearestInt(),
+                    juce::Justification::centred, false);
+    }
+
     juce::Path path;
     path.startNewSubPath (b.getX() + 4.0f, midY);
     const int n = 256;
@@ -99,7 +95,7 @@ void LFODisplay::editAt (float x, float y)
     const float v = juce::jlimit (-1.0f, 1.0f, (midY - y) / (b.getHeight() * 0.42f));
 
     if (shapeData.empty())
-        shapeData.assign (64, 0.0f);
+        shapeData.assign ((size_t) LFO::kShapePoints, 0.0f);
 
     if (shape == (int) LfoShape::StepSeq)
     {