Γ-W1-T7: make Preserve's splices pitch-synchronous — the jump is a whole number of the source's own period, detected once at load

30 Hz out-of-band energy 15.45% -> 0.00%; the 29 Hz rate-2.0 detune -133 -> +0 cents.
An unknown period keeps the fixed-window geometry bit for bit. The detector cannot
reach process(): sampler_core does not link it.

src/core/instrument/engine/voice.cpp

@@ -241,6 +241,12 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
         stretch_.start(p);
         shiftL_.setFeedRate(stretchRate_);
         shiftR_.setFeedRate(stretchRate_);
+        // Pitch-synchronous splices: the period was detected once at load (period_detect,
+        // which this library deliberately does not link — the loader hands the answer down on
+        // SampleData). 0 restores the fixed-window geometry, so a capture with no single
+        // period plays exactly as it always did.
+        shiftL_.setSourcePeriod(sample.sourcePeriodFrames);
+        shiftR_.setSourcePeriod(sample.sourcePeriodFrames);
         if (!loopWrap && primeCount < w) {
             // Sub-window playable span: the source is already exhausted at prime time.
             shiftL_.freezeTail();