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/Resources/AGENTS.md

@@ -0,0 +1,34 @@
+# resources module
+
+## Owned files
+
+- Related embedded assets and helpers: `../Resources.h`, `../Resources.cpp`
+- logo.svg
+- background.svg
+
+## Rules
+
+- Keep every theme colour in `namespace theme` in Resources.h.
+- Embed SVG assets as raw string literals.
+- Keep formatting helpers (formatPercent, formatHz, formatSeconds, formatSemis, formatCents, formatPan, formatInt) here.
+- Use `maps::` from Params for unit conversions in formatters.
+- Never hardcode a theme colour in a widget; reference `theme::`.
+
+## IF-THEN
+
+- IF a widget needs a colour THEN reference a `theme::` entry rather than a literal Colour.
+- IF an SVG asset ships under Source/Resources THEN keep its embedded string in Resources.cpp consistent with the file.
+
+## Examples
+
+```cpp
+// BAD: hardcoded colour in a widget
+g.setColour (juce::Colour (0xff4fc3f7));
+```
+
+```cpp
+// GOOD: theme is the single source
+g.setColour (theme::accent);
+```
+
+This file overrides /AGENTS.md where they conflict.