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/GUI/FilterDisplay.cpp

@@ -1,8 +1,12 @@
 #include "FilterDisplay.h"
+#include "../ExactCompare.h"
 
 namespace serum
 {
 
+// Schematic frequency-response preview for this view only. It does not model the
+// real DSP in Source/Filter.cpp, nor the drive, the slope or modulation, so it is
+// not expected to match the nonlinear filter numerically.
 float FilterDisplay::magnitude (float freqHz, float cutoffHz, float res, int type)
 {
     const float w = (cutoffHz > 1.0f) ? freqHz / cutoffHz : 1.0f;
@@ -29,7 +33,9 @@ float FilterDisplay::magnitude (float freqHz, float cutoffHz, float res, int typ
             }
             return m / 3.0f;
         }
+        case FilterModel::LadderLP:
         case FilterModel::Diode:
+        case FilterModel::Count:
         default:
             return (denom > 1e-6f) ? 1.0f / denom : 0.0f;
     }
@@ -37,7 +43,8 @@ float FilterDisplay::magnitude (float freqHz, float cutoffHz, float res, int typ
 
 void FilterDisplay::setParams (int t, float c, float r, float d, int s)
 {
-    if (type == t && cutoff == c && res == r && drive == d && slope == s)
+    if (type == t && ! changed (cutoff, c) && ! changed (res, r)
+        && ! changed (drive, d) && slope == s)
         return;
     type = t; cutoff = c; res = r; drive = d; slope = s;
     repaint();