Γ-W1-T5 remediation: narrow the rate-bound claim, fix baseline/measurement provenance, correct §2.4 framing

Re-derives the splice-cadence inequality and adds a corner probe that FAILS at
P=500 by design, pending a ruling. Names the baseline commit and harness edit,
fixes measurement methodology, corrects the Trigger-AHD/rate coupling framing.

tests/test_pitch_shift.cpp

@@ -737,6 +737,38 @@ static void testStretchAndShiftComposeSafely() {
     }
 }
 
+// The [0.5, 2.0] rate bound (time_stretch.h) narrows the splice-cadence failure onto the
+// source fundamental rather than eliminating it. At rate 2.0, shift 0.25 (-24 st) — both
+// inside the shipped range — the header's own derivation puts the safe-source floor at a
+// period of 315 frames (~140 Hz @ 44.1k): testStretchAndShiftComposeSafely's probe period of
+// 196.37 frames (~225 Hz) sits ABOVE that floor, so it passes because of the probe, not
+// because of headroom. This probe sits BELOW the floor on purpose, asserting the corner
+// rather than assuming it. A failure here is the inequality's PREDICTED outcome, not a
+// defect this test exists to chase — report it, don't retune the tolerance to hide it.
+static void testStretchCadenceBelowSafeFloorAtRate2ShiftQuarter() {
+    const std::int64_t w = 2205;
+    const double rate = 2.0;
+    const double shift = std::pow(2.0, -24.0 / 12.0);  // 0.25
+    for (double period : {500.0, 600.0, 700.0}) {
+        const double f0 = 1.0 / period;
+        const std::size_t srcLen = 400000;
+        std::vector<AudioSample> src(srcLen);
+        for (std::size_t i = 0; i < srcLen; ++i) {
+            src[i] = static_cast<AudioSample>(std::sin(2.0 * kPi * f0 * static_cast<double>(i)));
+        }
+        const std::size_t outFrames = 60000;
+        const std::vector<double> out = runStretch(src, w, rate, shift, outFrames, nullptr);
+        for (double v : out) CHECK(std::isfinite(v));
+        const double p = periodIn(out, 20000, 50000);
+        const double want = period / shift;
+        const bool ok = approx(p, want, want * 0.12);
+        std::printf("  [floor probe] period %.0f (rate 2.0, -24 st): observed %.2f want %.2f "
+                    "-> %s\n", period, p, want, ok ? "held" : "FAILED (predicted by the "
+                    "inequality in time_stretch.h)");
+        CHECK(ok);
+    }
+}
+
 // The two new entry points on a shifter that was never configured (a Varispeed voice's) —
 // neither may touch the empty ring.
 static void testStretchEntryPointsOnPassThrough() {
@@ -758,6 +790,7 @@ int main() {
     testStereoLinkedLagSharedSchedule();
     testStretchMovesDurationNotPitch();
     testStretchAndShiftComposeSafely();
+    testStretchCadenceBelowSafeFloorAtRate2ShiftQuarter();
     testStretchEntryPointsOnPassThrough();
 
     if (g_fail == 0) {