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/Oscillator.h

@@ -42,7 +42,7 @@ public:
     void reset();
 
     // Initialise a fresh note's unison sub-voices: phase offsets, detune, pan and gain.
-    void noteOn (double freqHz, const OscParams& p, juce::uint32 seed);
+    void noteOn (const OscParams& p, juce::uint32 seed);
     void setParams (const OscParams& p) noexcept;
 
     // Accumulate this oscillator's contribution into outL/outR.