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

.claude/helpers/graft-statusline.cjs

@@ -64,4 +64,4 @@ function entry(name) {
   return path.join(dir, 'dist', 'claude', name); // last-ditch; import will no-op if absent
 }
 
-import(pathToFileURL(entry("statusline.js")).href).then((m) => m.main()).catch(() => { /* graft unavailable — no-op */ });
+import(pathToFileURL(entry("statusline.js")).href).then((m) => m.main()).catch(() => { /* graft unavailable, no-op */ });