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.

src/core/instrument/engine/time_stretch.h

@@ -11,13 +11,22 @@
 namespace reasampler::instrument::engine {
 
 // The playback rates the Preserve DSP is measured over, and therefore the only ones it
-// accepts. Two independent reasons they are here and not wider:
-//  - the ceiling is what bounds a voice's per-output-frame feed loop (kMaxFeedPerFrame source
-//    frames), which is the RT-safety argument for feeding a variable count at all;
-//  - the splice search can only align a period it can see. The tap's delay drifts at
-//    |rate - shift| per frame, so a wide rate over a deep DOWN-shift splices faster than one
-//    period of the output tone and the correlation stops holding the pitch: measured at rate
-//    4.0 with -24 st, the observed period came out 539 frames against 785 wanted.
+// accepts. The ceiling also bounds a voice's per-output-frame feed loop (kMaxFeedPerFrame
+// source frames) — the RT-safety argument for feeding a variable count at all.
+//
+// This range NARROWS the splice-cadence failure onto the source fundamental; it does not
+// eliminate it. A splice recurs every `window / |rate - shift|` output frames (the tap's
+// delay drifts across one window at that per-frame rate); the shifted tone's own period is
+// `sourcePeriod / shift` output frames. Whenever the recurrence interval is shorter than
+// that period, a splice lands inside a single perceived cycle and the correlation search
+// 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.)
 inline constexpr double kStretchRateMin = 0.5;
 inline constexpr double kStretchRateMax = 2.0;
 inline constexpr int kMaxFeedPerFrame = 2;  // ceil(kStretchRateMax)