Fix cross-rate Preserve splices, re-stamp on host-rate change, name the rate refusal

The splice crossfade now counts output frames and stops before the ring's far end (this also fixes a lap at matched rate). The search strides by the rate ratio. The editor takes its time base from its own decode. Same-rate output is pinned to hashes from before the change.

src/core/instrument/engine/voice.h

@@ -556,9 +556,10 @@ private:
             shiftL_.setShiftRatio(shift);
             if (stereoOut) shiftR_.setShiftRatio(shift);
 
-            // 0..kMaxFeedPerFrame source frames fall due this output frame. All but the LAST are
-            // written without producing output; the last rides the ordinary 1-in-1-out
-            // process(), so a rate of exactly 1.0 walks the pre-stretch code path unchanged.
+            // 0..ceil(kStretchRateMax x sourceRatio_) source frames fall due this output frame
+            // (StretchCursor::due). All but the LAST are written without producing output; the
+            // last rides the ordinary 1-in-1-out process(), so a feed of exactly 1.0 walks the
+            // pre-stretch code path unchanged.
             // Crossfaded on the way IN to the shifter, not on the way out: loop the source,
             // shift the output.
             const std::int64_t due = stretch_.due(stretchRate_);