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/Filter.h

@@ -14,7 +14,7 @@ namespace serum
 class Filter
 {
 public:
-    void prepare (double sampleRate, int maxBlockSize);
+    void prepare (double sampleRate);
     void reset();
 
     // Process a mono buffer in-place. cutoffNorm is 0..1 (mapped to 20Hz..20kHz).
@@ -43,6 +43,7 @@ private:
     std::vector<float> combLine;
     int    combWrite = 0;
     double combDamp  = 0.0;
+    bool   combDirty = false;   // set once comb() has written to combLine
 
     // Formant: three parallel bandpass SVFs (state pairs).
     std::array<std::array<double, 2>, 3> formantState { { { { 0.0, 0.0 } }, { { 0.0, 0.0 } }, { { 0.0, 0.0 } } } };
@@ -50,9 +51,7 @@ private:
 
     void   updateCoefficients (float cutoffHz, float res, int type) noexcept;
     void   updateSvf (double g, double k) noexcept;
-    double svfLow   (double in, double g, double k) noexcept;
     double svfBand  (double in, double g, double k) noexcept;
-    double svfHigh  (double in, double g, double k) noexcept;
     double ladder   (double in, double g, double res, double drive, int stages, bool diode) noexcept;
     double comb     (double in, double freqHz, double res, double drive) noexcept;
     double formant  (double in) noexcept;