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

CMakeLists.txt

@@ -53,6 +53,7 @@ set(SERUMALT_SOURCES
     Source/EffectUnits/Delay.cpp
     Source/EffectUnits/Reverb.cpp
     Source/GUI/Knob.cpp
+    Source/GUI/KnobLookAndFeel.cpp
     Source/GUI/Slider.cpp
     Source/GUI/ToggleButton.cpp
     Source/GUI/Display.cpp