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; }
 };

src/core/instrument/engine/time_stretch.h

@@ -50,7 +50,11 @@ namespace reasampler::instrument::engine {
 // The fix is not a wider window: it is a nominal jump that is a whole number of the source's
 // own periods, so an aligned landing point exists by construction (pitch_shift.h's
 // periodAlignedJump, fed by period_detect at load). The same measurements then read 0.00% and
-// 0.00%, and 29 Hz renders at +0.0 cents. What survives: a period longer than the reachable
+// 0.00%, and 29 Hz renders at +0.0 cents — all from `preserve_low_frequency_tests` (Release,
+// hand-run; it is not in the gated ctest set), the same harness/config as the 3.6%/15.5%/-133
+// cents readings above. The gated suite's own number for this is the floor-relative excess in
+// pitch_shift_tests' testThirtyHertzSplicesAlignOnceTheSourcePeriodIsKnown, a different
+// quantity from the raw percentages here. What survives: a period longer than the reachable
 // jump (~1.25 windows, so below ~16 Hz at 50 ms) still cannot align, and a source with no
 // single period falls back to this fixed-window geometry by design.
 inline constexpr double kStretchRateMin = 0.5;