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/EnvelopeDisplay.cpp

@@ -1,11 +1,14 @@
 #include "EnvelopeDisplay.h"
+#include "../Envelope.h"
+#include "../ExactCompare.h"
 
 namespace serum
 {
 
 void EnvelopeDisplay::setParams (float a, float d, float s, float r, float c)
 {
-    if (attack == a && decay == d && sustain == s && release == r && curve == c)
+    if (! changed (attack, a) && ! changed (decay, d) && ! changed (sustain, s)
+        && ! changed (release, r) && ! changed (curve, c))
         return;
     attack = a; decay = d; sustain = s; release = r; curve = c;
     repaint();
@@ -19,8 +22,8 @@ void EnvelopeDisplay::paint (juce::Graphics& g)
     g.setColour (theme::outline);
     g.drawRoundedRectangle (b.expanded (4.0f).reduced (0.5f), 4.0f, 1.0f);
 
-    const float atkShape = 0.3f + curve * 2.7f;
-    const float decShape = 3.0f - curve * 2.7f;
+    const float atkShape = attackShapeFor (curve);
+    const float decShape = decayShapeFor (curve);
 
     // Normalise durations for display, allowing a short sustain plateau.
     const float aSec = maps::toSeconds (attack);