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/params/AGENTS.md

@@ -0,0 +1,43 @@
+# params module
+
+## Related source files
+
+This directory contains guidance; the implementation remains in `../Params.h`
+and `../Params.cpp`.
+
+## Rules
+
+- Use the `ids::` namespace for every APVTS parameter ID string.
+- Require one `ids::` entry per audio parameter, and one entry only.
+- Reach indexed parameters through the `paramIds::` arrays in `../Params.h`, which pair each
+  ID with its slot: `paramIds::oscillators`, `paramIds::envAttack`, `paramIds::lfoRate`,
+  `paramIds::fxType`, `paramIds::macros` and their siblings. Use a single `ids::` entry only
+  when the parameter has no index.
+- Use `enum class` for every enum. Give each a trailing `Count` enumerator.
+- Use `inline constexpr` for parameter ID strings and `k`-prefixed integer constants.
+- Keep all 0..1 to physical unit conversions in the `maps::` namespace.
+- Keep the wavetable name table (`kWavetableNames`) in sync with `kNumWavetables`.
+- Return an empty `juce::String` from enum-to-string switches for unknown values.
+- Never re-declare a parameter ID string literal outside Params.h.
+- Never add a physical unit mapping anywhere but `maps::`.
+
+## IF-THEN
+
+- IF you add an audio parameter THEN add its ID to `ids::` and, when it has physical units, add its mapping to `maps::` before any engine or GUI code reads it.
+- IF a switch converts an enum to a string THEN include a default case that returns an empty string.
+
+## Examples
+
+```cpp
+// BAD: re-declares the ID and drifts from Params.h
+if (auto* p = apvts.getParameter ("f1Cutoff"))
+    return p->getValue();
+```
+
+```cpp
+// GOOD: one source of truth
+if (auto* p = apvts.getParameter (ids::f1Cutoff))
+    return p->getValue();
+```
+
+This file overrides /AGENTS.md where they conflict.