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/KnobLookAndFeel.cpp

@@ -0,0 +1,47 @@
+#include "KnobLookAndFeel.h"
+
+namespace serum
+{
+
+void KnobLookAndFeel::drawRotarySlider (juce::Graphics& g, int x, int y, int width, int height,
+                                        float sliderPos, float rotaryStartAngle, float rotaryEndAngle,
+                                        juce::Slider& slider)
+{
+    const auto dial = juce::Rectangle<float> ((float) x, (float) y, (float) width, (float) height);
+    if (dial.isEmpty())
+        return;
+
+    const float radius = juce::jmin (dial.getWidth(), dial.getHeight()) * 0.5f - 2.0f;
+    if (radius <= 1.0f)
+        return;
+
+    const float lineW  = juce::jlimit (1.5f, 5.0f, radius * 0.22f);
+    const float arcR   = radius - lineW * 0.5f;
+    const float centreX = dial.getCentreX();
+    const float centreY = dial.getCentreY();
+    const float toAngle = rotaryStartAngle + sliderPos * (rotaryEndAngle - rotaryStartAngle);
+    const bool  live    = slider.isEnabled();
+
+    juce::Path track;
+    track.addCentredArc (centreX, centreY, arcR, arcR, 0.0f, rotaryStartAngle, rotaryEndAngle, true);
+    g.setColour (theme::outline);
+    g.strokePath (track, juce::PathStrokeType (lineW, juce::PathStrokeType::curved, juce::PathStrokeType::rounded));
+
+    if (live)
+    {
+        juce::Path value;
+        value.addCentredArc (centreX, centreY, arcR, arcR, 0.0f, rotaryStartAngle, toAngle, true);
+        g.setColour (theme::accent.withAlpha (slider.isMouseOverOrDragging() ? 1.0f : 0.75f));
+        g.strokePath (value, juce::PathStrokeType (lineW, juce::PathStrokeType::curved, juce::PathStrokeType::rounded));
+    }
+
+    const float pointer = radius * 0.62f;
+    g.setColour (live ? theme::text : theme::textDim);
+    g.drawLine (centreX + std::sin (toAngle) * pointer * 0.35f,
+                centreY - std::cos (toAngle) * pointer * 0.35f,
+                centreX + std::sin (toAngle) * pointer,
+                centreY - std::cos (toAngle) * pointer,
+                juce::jmax (1.5f, lineW * 0.5f));
+}
+
+} // namespace serum