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.

tests/test_sampler_core.cpp

@@ -3214,7 +3214,10 @@ static void testPreserveStretchThirtyTwoVoicesHoldUp() {
     const std::size_t blockFrames = 44100;        // one second of audio
     const std::size_t voiceCount = 32;
     const int kWarmupReps = 2;
-    const int kTimedReps = 7;
+    // 5 rather than 7: the source-period rows below doubled the row count, and run-to-run
+    // spread on this machine is ~5% either way, so the extra reps bought precision the number
+    // does not carry while costing the Debug gate real seconds.
+    const int kTimedReps = 5;
     SampleData s = stretchProbeSample(200000, true);
     s.loop.hasLoop = true;                        // held notes: all 32 sound for the whole run
     s.loop.start = 40000;
@@ -3223,7 +3226,24 @@ static void testPreserveStretchThirtyTwoVoicesHoldUp() {
 
     // 1.0 is the reference: it is the cost the shipped Preserve read already carries, so the
     // two stretched rows are read as a delta against it rather than in isolation.
-    for (double rate : {1.0, 0.5, 2.0}) {
+    //
+    // The last two rows carry a SOURCE PERIOD, which is where the pitch-synchronous splice can
+    // cost something: the jump becomes a whole number of periods, and when that is shorter than
+    // the window the splice cadence rises in proportion — more correlation searches per second.
+    // 1470 (30 Hz at 44.1k) is the worst realistic case in the audible band, forcing a jump of
+    // 2/3 the window and therefore 1.5x the searches. 220.5 (200 Hz) is the typical one: ten
+    // periods land exactly on the window, so the cadence is unchanged and the row should read
+    // as the no-period one — which is the measurement that separates "the mechanism costs
+    // something" from "a shorter jump costs something". There is no per-frame cost either way:
+    // the jump is resolved once at note-on.
+    struct Row { double rate; double period; };
+    const Row rows[] = {
+        {1.0, 0.0}, {0.5, 0.0}, {2.0, 0.0},
+        {1.0, 220.5}, {1.0, 1470.0}, {2.0, 1470.0},
+    };
+    for (const Row& row : rows) {
+        const double rate = row.rate;
+        s.sourcePeriodFrames = row.period;
         std::vector<double> nsPerVoiceFrame;
         nsPerVoiceFrame.reserve(kTimedReps);
         for (int rep = 0; rep < kWarmupReps + kTimedReps; ++rep) {
@@ -3263,10 +3283,11 @@ static void testPreserveStretchThirtyTwoVoicesHoldUp() {
         const double medianNs = nsPerVoiceFrame[nsPerVoiceFrame.size() / 2];
         const double secsAtMedian =
             medianNs * static_cast<double>(blockFrames) * static_cast<double>(voiceCount) / 1e9;
-        std::printf("  [measure] 32 stereo Preserve voices @ rate %.2f: median %.1f ns/voice/"
-                    "frame [%.1f .. %.1f] over %d reps (%.1f%% of realtime at the median)\n",
-                    rate, medianNs, nsPerVoiceFrame.front(), nsPerVoiceFrame.back(), kTimedReps,
-                    100.0 * secsAtMedian);
+        std::printf("  [measure] 32 stereo Preserve voices @ rate %.2f, source period %6.1f: "
+                    "median %.1f ns/voice/frame [%.1f .. %.1f] over %d reps (%.1f%% of realtime "
+                    "at the median)\n",
+                    rate, row.period, medianNs, nsPerVoiceFrame.front(), nsPerVoiceFrame.back(),
+                    kTimedReps, 100.0 * secsAtMedian);
     }
 }