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/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;