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/SubOscillator.cpp

@@ -3,12 +3,8 @@
 namespace serum
 {
 
-void SubOscillator::noteOn (double freqHz, int octaveShift)
+void SubOscillator::noteOn()
 {
-    // octaveShift: -2, -1 or 0 (from the subOct param).
-    const double mult = (octaveShift == -2) ? 0.25 : (octaveShift == -1) ? 0.5 : 1.0;
-    (void) freqHz;
-    (void) mult;
     phase = 0.0;
 }
 
@@ -18,14 +14,14 @@ void SubOscillator::processAdd (double freqHz, int shape, float level, float& ou
         return;
 
     // The octave shift is baked into freqHz by the voice; here we just phase-accumulate.
-    const double inc = kTwoPi * freqHz / sr;
+    const double inc = juce::MathConstants<double>::twoPi * freqHz / sr;
     phase += inc;
-    phase -= std::floor (phase * (1.0 / kTwoPi)) * kTwoPi;
+    phase -= std::floor (phase * (1.0 / juce::MathConstants<double>::twoPi)) * juce::MathConstants<double>::twoPi;
 
     float s = 0.0f;
     if (shape == (int) SubShape::Triangle)
     {
-        const float p = (float) (phase * (1.0 / kTwoPi));
+        const float p = (float) (phase * (1.0 / juce::MathConstants<double>::twoPi));
         s = 1.0f - 4.0f * std::abs (p - 0.5f);
     }
     else