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/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));