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/Slider.cpp

@@ -3,8 +3,8 @@
 namespace serum
 {
 
-Slider::Slider (const juce::String& name, std::function<juce::String (float)> formatter)
-    : Knob (name, std::move (formatter))
+Slider::Slider (const juce::String& name, std::function<juce::String (float)> fmt)
+    : Knob (name, std::move (fmt))
 {
     setSliderStyle (juce::Slider::LinearHorizontal);
     setTextBoxStyle (juce::Slider::NoTextBox, false, 0, 0);