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/ExactCompare.h

@@ -0,0 +1,42 @@
+#pragma once
+
+// Exact floating-point comparison helpers.
+//
+// The engine caches derived state - filter coefficients, per-voice parameter
+// snapshots, GUI preview values - and re-derives it only when a stored value
+// changes. Those checks must stay bit-exact: an epsilon compare would either
+// keep stale cached state or skip a needed rebuild near the threshold, so the
+// intent is named here instead of weakened at each call site.
+//
+// Every first-party use of == / != on floating point goes through one of these
+// helpers, and the single -Wfloat-equal suppression lives here rather than
+// being repeated across the call sites.
+
+#include <cmath>
+
+#if defined(__GNUC__) || defined(__clang__)
+ #pragma GCC diagnostic push
+ #pragma GCC diagnostic ignored "-Wfloat-equal"
+#endif
+
+namespace serum
+{
+
+// Exact equality, no epsilon. Used where the exact value carries meaning, such
+// as "the output level is exactly unity" or "the untouched tail is the zero
+// default".
+inline bool exactly (float a, float b) noexcept { return a == b; }
+inline bool exactly (double a, double b) noexcept { return a == b; }
+
+// Exact inequality, for "the cached value changed" tests.
+inline bool changed (float a, float b) noexcept { return a != b; }
+inline bool changed (double a, double b) noexcept { return a != b; }
+
+// True when a stored double that should hold a whole number really does.
+inline bool isWholeNumber (double v) noexcept { return std::floor (v) == v; }
+
+} // namespace serum
+
+#if defined(__GNUC__) || defined(__clang__)
+ #pragma GCC diagnostic pop
+#endif