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.h

@@ -0,0 +1,34 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "../Resources.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Look-and-feel for Knob.
+//
+// Two jobs:
+//
+//  1. Draw the rotary with theme:: colours. The shared default look-and-feel
+//     paints it with its own palette, which ignores the rest of the editor.
+//
+//  2. Leave room for the knob's name. juce::LookAndFeel_V4 reduces the rotary
+//     bounds by a fixed ten pixels on each side, so a 76 pixel knob loses
+//     20 pixels of dial and a smaller knob loses most of it. This version
+//     keeps the dial whole inside its footprint and reserves a text strip
+//     under it, which is the strip Knob paints its name into.
+// ===========================================================================
+class KnobLookAndFeel : public juce::LookAndFeel_V4
+{
+public:
+    // Height of the name strip reserved under the dial.
+    static constexpr int labelHeight = 16;
+
+    void drawRotarySlider (juce::Graphics& g, int x, int y, int width, int height,
+                           float sliderPos, float rotaryStartAngle, float rotaryEndAngle,
+                           juce::Slider& slider) override;
+};
+
+} // namespace serum