The rail dim (and its top fade band) was clipped to the rail's footprint,
so it terminated in a vertical seam at the nav edge while the backdrop
continued behind the nav. Generalize _RailBackgroundBleed into
_RailBleedPositioned and render the dim through it, full-width like the
background gradient. Row stripes now apply only while the rail itself is
focused — under the full-width dim they'd re-introduce level seams at the
nav edge (and read as noise anyway).
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