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

@@ -9,6 +9,10 @@ and `../Params.cpp`.
 
 - 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.
@@ -36,4 +40,4 @@ if (auto* p = apvts.getParameter (ids::f1Cutoff))
     return p->getValue();
 ```
 
-This file overrides /AGENT.md where they conflict.
+This file overrides /AGENTS.md where they conflict.