transform fails to re-run when file loaded at different rate

Load a file at one sample rate (say 48000).

Run a plugin that requires an exact step size based on the rate (e.g. QM segmenter).

Load another file at a different rate (say 44100) to replace the main model.

The plugin fails to re-run because SV is trying to give it the same step size as before, when it wants a new step size to go with the new rate.

Fix: make SV remember not just the particular step size used, but the fact that that was the default, and then use the new default instead of reusing the old size.