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.cpp

@@ -41,8 +41,8 @@ void Envelope::setParams (float attackSec, float decaySec, float sustainLevel, f
     releaseSamples = std::max (1.0, (double) releaseSec * sr);
 
     // curve 0 -> fast attack / long release curve; curve 1 -> slow attack / fast decay
-    attackShape = 0.3 + curve * 2.7;    // 0.3 .. 3.0
-    decayShape  = 3.0 - curve * 2.7;    // 3.0 .. 0.3
+    attackShape = (double) attackShapeFor (curve);
+    decayShape  = (double) decayShapeFor (curve);
 
     shape.attack  = attackSec;
     shape.decay   = decaySec;