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/SynthVoice.h

@@ -72,6 +72,7 @@ public:
     bool isReleased() const noexcept   { return released; }
     int  getNote() const noexcept      { return note; }
     juce::uint64 getNoteId() const noexcept { return noteId; }
+    float getModulationValue (ModSource source) const noexcept;
 
 private:
     Oscillator     oscA, oscB;