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/synth/AGENTS.md

@@ -3,8 +3,7 @@
 ## Related source files
 
 This directory contains guidance. All files listed below remain in the parent
-`Source/` directory. The preparation rules below are requirements for changes,
-not a claim that lazy wavetable allocation has already been fixed.
+`Source/` directory.
 
 - Wavetable.h, Wavetable.cpp
 - Oscillator.h, Oscillator.cpp
@@ -26,10 +25,11 @@ not a claim that lazy wavetable allocation has already been fixed.
 - Use `(size_t)` casts when indexing containers with int loop variables.
 - Pass global per-block state into voices as a `const RenderContext&`.
 - Never allocate memory in the audio path.
+- Keep oscillator phase when changing held-note parameters. LFO phase offsets are separate from the running phase.
 
 ## IF-THEN
 
-- IF a wavetable is read in the render path THEN build all tables in `prepare()` by calling `WavetableLibrary::prebuild()` before the first block.
+- Wavetables are prebuilt during Engine construction. Audio and GUI code read them without triggering generation.
 - IF a parameter is read every block THEN snapshot it into a struct (`OscParams`, `RenderContext`) instead of reading the APVTS per sample.
 
 ## Examples
@@ -40,8 +40,8 @@ const Wavetable& wt = library.getTable (wave);
 ```
 
 ```cpp
-// GOOD: build once at prepare time, then only read const references
-library.prebuild();                 // in prepareToPlay
+// GOOD: build once in the Engine constructor, then only read const references
+library.prebuild();                 // in the Engine constructor
 const Wavetable& wt = library.getTable (wave);   // lookup, no allocation
 ```
 
@@ -55,4 +55,4 @@ const double step = 6.28318530717958647692 * freq / sr;
 const double step = juce::MathConstants<double>::twoPi * freq / sr;
 ```
 
-This file overrides /AGENT.md where they conflict.
+This file overrides /AGENTS.md where they conflict.