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/Knob.h

@@ -3,13 +3,16 @@
 #include <JuceHeader.h>
 #include <memory>
 #include "../Resources.h"
+#include "KnobLookAndFeel.h"
 
 namespace serum
 {
 
 // ===========================================================================
-// Rotary knob — a juce::Slider styled by SerumLookAndFeel. The value readout is
-// formatted by a user-provided function via getTextFromValue().
+// Rotary knob — a juce::Slider drawn by KnobLookAndFeel. The knob paints its
+// own name below the dial, so no control is a bare arc whose only hint is a
+// tooltip. The value readout is formatted by a user-provided function via
+// getTextFromValue().
 //
 // While the knob is being dragged it also shows a small floating label near the
 // mouse cursor with the live (formatted) value, giving immediate feedback
@@ -23,8 +26,16 @@ public:
 
     void setFormatter (std::function<juce::String (float)> f) { formatter = std::move (f); }
 
+    // Value a double click returns to. Call it with the parameter's own default
+    // so the gesture restores the real default, never a hardcoded 0.5.
+    void setDefaultValue (double v) { setDoubleClickReturnValue (true, v); }
+
     juce::String getTextFromValue (double value) override;
 
+    void paint (juce::Graphics& g) override;
+    void resized() override;
+    void visibilityChanged() override;
+
     void mouseDown (const juce::MouseEvent& e) override;
     void mouseDrag (const juce::MouseEvent& e) override;
     void mouseUp   (const juce::MouseEvent& e) override;
@@ -36,6 +47,7 @@ private:
 
     std::function<juce::String (float)> formatter;
     std::unique_ptr<juce::Label> valuePopup;
+    juce::Rectangle<int> textArea;   // name strip under the dial
 };
 
 } // namespace serum