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/SubOscillator.h

@@ -15,7 +15,7 @@ public:
     void prepare (double sampleRate)  { sr = sampleRate; phase = 0.0; }
     void reset()                       { phase = 0.0; }
 
-    void noteOn (double freqHz, int octaveShift);
+    void noteOn();
 
     // Accumulate into out (mono; the caller pans/levels it).
     void processAdd (double freqHz, int shape, float level, float& out) noexcept;
@@ -23,7 +23,6 @@ public:
 private:
     double sr    = 44100.0;
     double phase = 0.0;
-    static constexpr double kTwoPi = 6.28318530717958647692;
 };
 
 } // namespace serum