Fix inverted splice-cadence test: assert artifact energy, not zero-crossing period

Zero-crossing counting was anti-correlated with the real defect (splice debris
fools it). Now asserts energy outside the fundamental, with an alignable control,
matching test_preserve_low_frequency.cpp's approach.

src/core/instrument/engine/time_stretch.h

@@ -22,11 +22,15 @@ namespace reasampler::instrument::engine {
 // has less than one period to align against. Measured at rate 4.0, shift 0.25 (-24 st):
 // interval 2205/3.75 ~= 588 vs period ~4*P ~= 785 frames (P ~= 196) — matches the originally
 // observed 539-vs-785 failure. This range's ceiling (2.0, not 4.0) raises the safe floor, it
-// does not remove it: at rate 2.0, shift 0.25, interval = 2205/1.75 = 1260 still fails for
-// any source period P > 315 frames (~140 Hz at 44.1k) — inside bass/low-vocal material, and
-// -24 st is reachable from the Pitch knob alone. (The pre-stretch rate-1.0 engine's floor by
-// the same inequality is P > 735, ~60 Hz — what this range raises the floor from, not what
-// it removes.)
+// does not remove it: at rate 2.0, shift 0.25, interval = 2205/1.75 = 1260 still produces
+// measurable splice debris for any source period P > 315 frames (~140 Hz at 44.1k) — inside
+// bass/low-vocal material, and -24 st is reachable from the Pitch knob alone. pitch_shift_tests
+// (testStretchCadenceCornerArtifactEnergyAtRate2ShiftQuarter) asserts this corner directly at
+// P=500/600/700: energy outside the fundamental runs 7-21% there against ~0% on an aligned
+// control at the same rate/shift — zero-crossing period is NOT what it checks, since splice
+// debris fools that estimator into reading the wrong period on a render whose fundamental is
+// actually fine. (The pre-stretch rate-1.0 engine's floor by the same inequality is P > 735,
+// ~60 Hz — what this range raises the floor from, not what it removes.)
 //
 // A SECOND, INDEPENDENT limit binds the same material, and no rate bound touches it. A splice
 // relocates the tap by the nominal window refined by a search over +/- window/4, so the