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/EffectUnits/AGENTS.md

@@ -50,4 +50,4 @@ void prepare (double sampleRate, int) override
 }
 ```
 
-This file overrides /AGENT.md where they conflict.
+This file overrides /AGENTS.md where they conflict.

Source/EffectUnits/Biquad.h

@@ -28,7 +28,7 @@ public:
 
     void setLowpass (double freq, double q)
     {
-        const double w0 = 2.0 * juce::MathConstants<double>::pi * freq / sampleRate;
+        const double w0 = juce::MathConstants<double>::twoPi * freq / sampleRate;
         const double cw = std::cos (w0);
         const double sw = std::sin (w0);
         const double alpha = sw / (2.0 * q);
@@ -42,7 +42,7 @@ public:
 
     void setHighpass (double freq, double q)
     {
-        const double w0 = 2.0 * juce::MathConstants<double>::pi * freq / sampleRate;
+        const double w0 = juce::MathConstants<double>::twoPi * freq / sampleRate;
         const double cw = std::cos (w0);
         const double sw = std::sin (w0);
         const double alpha = sw / (2.0 * q);
@@ -57,7 +57,7 @@ public:
     void setLowShelf (double freq, double gainDb)
     {
         const double a = std::pow (10.0, gainDb / 40.0);
-        const double w0 = 2.0 * juce::MathConstants<double>::pi * freq / sampleRate;
+        const double w0 = juce::MathConstants<double>::twoPi * freq / sampleRate;
         const double cw = std::cos (w0);
         const double sw = std::sin (w0);
         const double alpha = sw / 2.0 * std::sqrt (2.0);
@@ -72,7 +72,7 @@ public:
     void setHighShelf (double freq, double gainDb)
     {
         const double a = std::pow (10.0, gainDb / 40.0);
-        const double w0 = 2.0 * juce::MathConstants<double>::pi * freq / sampleRate;
+        const double w0 = juce::MathConstants<double>::twoPi * freq / sampleRate;
         const double cw = std::cos (w0);
         const double sw = std::sin (w0);
         const double alpha = sw / 2.0 * std::sqrt (2.0);
@@ -87,7 +87,7 @@ public:
     void setPeak (double freq, double gainDb, double q)
     {
         const double a = std::pow (10.0, gainDb / 40.0);
-        const double w0 = 2.0 * juce::MathConstants<double>::pi * freq / sampleRate;
+        const double w0 = juce::MathConstants<double>::twoPi * freq / sampleRate;
         const double cw = std::cos (w0);
         const double sw = std::sin (w0);
         const double alpha = sw / (2.0 * q);

Source/EffectUnits/Chorus.cpp

@@ -37,13 +37,13 @@ void ChorusUnit::process (float* l, float* r, int numSamples, const float p[4])
         delayL[(size_t) writePos] = inL;
         delayR[(size_t) writePos] = inR;
 
-        lfoPhase += 6.28318530717958647692 * rate / sr;
-        if (lfoPhase > 6.28318530717958647692) lfoPhase -= 6.28318530717958647692;
+        lfoPhase += juce::MathConstants<double>::twoPi * rate / sr;
+        if (lfoPhase > juce::MathConstants<double>::twoPi) lfoPhase -= juce::MathConstants<double>::twoPi;
 
         float sumL = 0.0f, sumR = 0.0f;
         for (int v = 0; v < 2; ++v)
         {
-            const double mod = std::sin (lfoPhase + v * 3.14159265358979323846) * depth;
+            const double mod = std::sin (lfoPhase + v * juce::MathConstants<double>::pi) * depth;
             const double d = base + mod + (v == 1 ? width * depth * 0.5 : 0.0);
             double readPos = writePos - d;
             if (readPos < 0.0) readPos += len;

Source/EffectUnits/Flanger.cpp

@@ -35,12 +35,16 @@ void FlangerUnit::process (float* l, float* r, int numSamples, const float p[4])
     {
         const float inL = l[i], inR = r[i];
 
-        lfoPhase += 6.28318530717958647692 * rate / sr;
-        if (lfoPhase > 6.28318530717958647692) lfoPhase -= 6.28318530717958647692;
+        lfoPhase += juce::MathConstants<double>::twoPi * rate / sr;
+        if (lfoPhase > juce::MathConstants<double>::twoPi) lfoPhase -= juce::MathConstants<double>::twoPi;
         const double mod = std::sin (lfoPhase) * depth;
 
+        // base + mod is not guaranteed positive: at high depth the LFO offset
+        // can exceed the centre delay, so the read position runs ahead of the
+        // write position and needs the wrap in both directions to stay in range.
         double readPos = writePos - (base + mod);
         if (readPos < 0.0) readPos += len;
+        if (readPos >= (double) len) readPos -= len;
         const int i0 = (int) readPos;
         const int i1 = (i0 + 1) % len;
         const float frac = (float) (readPos - std::floor (readPos));

Source/EffectUnits/Phaser.cpp

@@ -27,11 +27,15 @@ void PhaserUnit::process (float* l, float* r, int numSamples, const float p[4])
 
     for (int i = 0; i < numSamples; ++i)
     {
-        lfoPhase += 6.28318530717958647692 * rate / sr;
-        if (lfoPhase > 6.28318530717958647692) lfoPhase -= 6.28318530717958647692;
+        lfoPhase += juce::MathConstants<double>::twoPi * rate / sr;
+        if (lfoPhase > juce::MathConstants<double>::twoPi) lfoPhase -= juce::MathConstants<double>::twoPi;
 
         const float sweep = (float) (0.5 + 0.5 * std::sin (lfoPhase));
-        const float a = 0.3f + depth * sweep;   // allpass coefficient
+        // The allpass cascade is stable only while the coefficient stays inside
+        // (-1, 1). depth can push 0.3 + depth * sweep above 1, which makes every
+        // stage self-excite, so the filter state runs away to inf/NaN and keeps
+        // producing it. Clamp at the point where the coefficient is computed.
+        const float a = juce::jlimit (0.0f, 0.99f, 0.3f + depth * sweep);
 
         float outL = l[i] + yL[5] * feedback;
         float outR = r[i] + yR[5] * feedback;