_write and set_value both did a bare _value[int(item)] subscript while
the slider grid has validator.count entries. A persister value with
fewer keys than bands (stale data from an older EQ parser) raised
KeyError on render and on every slider drag for a missing band, so
slider changes never reached the persister.
Use .get() with a 0 dB fallback for unset bands, and guard against
_value not being a dict at all.
5853cf5a8e