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

@@ -0,0 +1,44 @@
+# plugin module
+
+## Related source files
+
+This directory contains guidance; the implementation remains in
+`../PluginProcessor.h` and `../PluginProcessor.cpp`.
+
+## Rules
+
+- Keep this the JUCE `AudioProcessor` boundary: APVTS, engine, RAVE, presets and state.
+- Declare the full parameter layout in `createParameterLayout()`.
+- Use `juce::ScopedNoDenormals` in `processBlock`.
+- 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`).
+- 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
+
+- IF state XML is null or invalid THEN return without touching the APVTS.
+- IF a parameter is missing from a preset load THEN skip it with an `if (auto* param = ...)` guard.
+
+## Examples
+
+```cpp
+// BAD: dereferences a possibly null XML result
+auto xml = getXmlFromBinary (data, size);
+auto state = juce::ValueTree::fromXml (*xml);
+```
+
+```cpp
+// GOOD: null guard, silent return
+std::unique_ptr<juce::XmlElement> xml (getXmlFromBinary (data, size));
+if (xml == nullptr)
+    return;
+juce::ValueTree state = juce::ValueTree::fromXml (*xml);
+if (! state.isValid())
+    return;
+```
+
+This file overrides /AGENTS.md where they conflict.