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/plugin/AGENTS.md

@@ -13,7 +13,10 @@ This directory contains guidance; the implementation remains in
 - Guard `getRawParameterValue` and `getParameter` results before dereferencing them.
 - Return silently on null or invalid state XML and invalid ValueTrees.
 - Use raw `new` only for JUCE-owned objects (`createEditor`, `createPluginFilter`).
-- Prefer const accessors over the public `parameters`, `engine` and `rave` fields for new code.
+- Hold the engine control lock across preset and state changes; preserve parameter IDs and normalized values for saved-project compatibility.
+- Factory presets reset sound parameters before applying overrides, while preserving UI scale.
+- Save one copy of each custom state section. LFO shape arrays are stored as JSON text inside XML.
+- RAVE is an ordinary parameter that enables a render-time boost; it does not rewrite other parameters.
 
 ## IF-THEN
 
@@ -38,4 +41,4 @@ if (! state.isValid())
     return;
 ```
 
-This file overrides /AGENT.md where they conflict.
+This file overrides /AGENTS.md where they conflict.