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/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;