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/engine/AGENTS.md

@@ -0,0 +1,50 @@
+# engine module
+
+## Related source files
+
+This directory contains guidance; the implementation remains in `../Engine.h`
+and `../Engine.cpp`.
+
+## Rules
+
+- Own the voice pool, LFOs, wavetable library, FX rack, matrix and macros by value.
+- Bind APVTS pointers in `prepare()`; `v()` reads captured values without constructing parameter-name strings.
+- Build the `RenderContext` once per host block and refresh its modulation inputs for each render segment.
+- Render up to 64 samples at a time, splitting at MIDI event offsets and respecting prepared buffer capacity.
+- Size `mixBuffer` once in `prepare()`; use non-owning views for shorter segments.
+- Prebuild wavetables in the Engine constructor, before either GUI painting or audio rendering can read them.
+- Clamp the final output with the soft limiter.
+- Never call `setSize` on any buffer inside `processBlock`.
+
+## IF-THEN
+
+- IF a per-block buffer is needed THEN allocate it in `prepare()` and clear it per block.
+- IF a module needs the matrix, macros or wavetables THEN go through Engine accessors rather than reaching into another module.
+
+## Examples
+
+```cpp
+// BAD: per-callback buffer reconfiguration on the audio thread
+void processBlock (...)
+{
+    mixBuffer.setSize (2, n, false, false, true);
+    ...
+}
+```
+
+```cpp
+// GOOD: size once, then wrap a non-owning view per segment
+void prepare (double sr, int blockSize)
+{
+    mixBuffer.setSize (2, blockSize, false, false, true);
+}
+void processBlock (...)
+{
+    // No allocation; the view only borrows mixBuffer's channels.
+    juce::AudioBuffer<float> block (mixBuffer.getArrayOfWritePointers(), 2, segmentLength);
+    block.clear();
+    ...
+}
+```
+
+This file overrides /AGENTS.md where they conflict.