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/map/sample_map.cpp

@@ -3,6 +3,8 @@
 
 #include "core/instrument/map/sample_map.h"
 
+#include "core/instrument/engine/period_detect.h"  // the load-time Preserve source period
+
 #include <algorithm>  // std::remove_if
 #include <cassert>    // assert
 #include <utility>    // std::move
@@ -325,6 +327,11 @@ SampleData buildSampleData(const ResolvedCapture& resolved, DecodedPcm decoded)
     // Resolve the stored wall-clock SECONDS (AHDSR, pitch env A/D) to frames at THIS WAV's
     // actual rate; source-timeline params (trigger %-length + fades, start) carry through.
     data.play = resolvePlay(resolved.play, data.sampleRate);
+    // The one place Preserve's source period is computed: the load, off the audio thread.
+    // Channel 0 only — a stereo pair's two channels share a fundamental, and the splice
+    // schedule is linked across them anyway.
+    data.sourcePeriodFrames =
+        instrument::engine::detectPeriod(data.frames, data.sampleRate).frames;
     return data;
 }