Gamma-W1-T7: gate PSOLA's two untested wires, fix the cadence test's contradictory rationale, add a vacuity guard

src/core/instrument/engine/period_detect.h

@@ -19,7 +19,10 @@ using audio::AudioSample;
 // authored and never persisted — this is a cache, not state.
 struct PeriodEstimate {
     double frames = 0.0;      // 0 = no single period (inharmonic, polyphonic, percussive, noise)
-    double confidence = 0.0;  // 1 - the accepted dissimilarity, [0,1]; 0 when frames == 0
+    // 1 - the accepted dissimilarity, [0,1]; 0 when frames == 0. Diagnostic only today: the
+    // accept decision is `valid()` alone, and the loader takes `.frames` without reading this —
+    // do not assume it is load-bearing without checking who reads it.
+    double confidence = 0.0;
 
     bool valid() const { return frames > 0.0; }
 };