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/LFO.h

@@ -62,4 +62,13 @@ private:
     float shapeValue() noexcept;
 };
 
+// ===========================================================================
+// Pure shape evaluation shared by the audio render (LFO::shapeValue) and the
+// GUI preview (LFODisplay::valueAt), so the drawn shape cannot drift from the
+// played one. `sampleHoldValue` supplies the current held level for
+// LfoShape::SampleHold, which is stateful in the audio path.
+// ===========================================================================
+float lfoShapeValue (float phase, LfoShape shape, int steps,
+                     const float* data, int dataSize, float sampleHoldValue) noexcept;
+
 } // namespace serum