fix(dirvish): stop webm preview thumbnails flashing blank

dirvish keys its media thumbnail cache on the exact preview-window pixel width (thumbnails/<width>/<md5>.jpg). Fast image thumbnails finish before the window settles, so their width is stable. Slow webm generation loses that race: the jpg is written at one width, then redisplay recomputes the width, and jitter across a floor boundary looks it up in a different bucket, so it misses, regenerates, and flashes blank.

I quantize the computed size to a 100px bucket (cj/dirvish-thumb-width-bucket) so jitter maps to one stable cache key. It's a filter-return advice on dirvish-media--img-size, added once dirvish-widgets loads. It's a local workaround for the upstream cache design. If the settle swing turns out larger than one bucket, widen the bucket or fall back to a width-independent hash.

The quantize math is unit-tested. The live preview render needs a manual check.

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