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/Envelope.h

@@ -5,6 +5,11 @@
 namespace serum
 {
 
+// Curve-to-shape mapping shared by the envelope DSP and the GUI preview, so the
+// two cannot drift apart.
+inline float attackShapeFor (float curve) noexcept { return 0.3f + curve * 2.7f; }  // 0.3 .. 3.0
+inline float decayShapeFor  (float curve) noexcept { return 3.0f - curve * 2.7f; }  // 3.0 .. 0.3
+
 // ===========================================================================
 // ADSR envelope with curve tension. Durations are in seconds; the sustain
 // level is 0..1. Retriggering (noteOn while active) starts from the current