G9: SelectionGlow, a soft halo round the body's selection

SelectionGlow on|off (off; a pixel shell's, Chrome.selection_glow): each
block of selected rows in a pane body casts Lift's soft kind in the
selection's own colour, the block its caster (never touched) and a cursor
beside it joined to the caster; clipped to the bodies (never a tag or a
grip); at most 22% (§8.2), less where the text round it would drop below
min(its contrast, 4.5); fading in over 100 ms with an ease-out, on the
core's continuous path (shell_continuous); none while panes move. Blocks
come from the cells in the theme's selection colour (documented).

Tests: the halo's colour, its clip to the body, nothing at its first frame
and up to its cap after 100 ms, fading done. Gates as G8. Feel review:
.scratch/render/selection/.

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