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.

tests/test_preserve_low_frequency.cpp

@@ -270,7 +270,7 @@ static SpliceStats spliceGeometry(const std::vector<AudioSample>& src, std::int6
     ps.setFeedRate(rate);
     ps.setSourcePeriod(sourcePeriod);
     StretchCursor cur;
-    cur.start(window);
+    cur.start(window, 1.0);
     loop::ResolvedLoop lp{};  // inactive: the source is long enough to run straight through
 
     SpliceStats st;