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/play_params.h

@@ -264,6 +264,14 @@ struct SampleData {
     // (never per frame). Default flat y=1 — every velocity plays at unity.
     VelocityCurve velocityCurve = VelocityCurve::flat();
 
+    // The source's own fundamental period in SOURCE frames, which makes Preserve's splices
+    // pitch-synchronous (pitch_shift.h). DERIVED from the PCM at load, not authored and never
+    // persisted — a cache, not state, so it takes no rung of the payload ladder. 0 means
+    // unknown (nothing detected it, or the source has no single period) and restores the
+    // fixed-window splice geometry byte for byte, which is why a hand-built SampleData is
+    // still exactly the bare engine.
+    double sourcePeriodFrames = 0.0;
+
     PlayParams play;
 
     // The live-parameter block a sounding voice tracks, or null for the bare latched engine