T1: ChipShadow, a terminal's notice chips cast a cell's shadow

ChipShadow on|off (off; the tty shell's, §9.2 T1): each notice chip casts
a shadow one cell right and one down inside its own pane's body: on a
blank cell a half block (▌ right, ▀ below, ▘ at the corner) in a colour
35% darker than the ground (OKLab), so the shadow is half a cell thick;
under text the ground itself darkened. Never the cursor's cell, a
selection, a tag or another chip; truecolor terminals only.

Tests: the shadow lies right of or below the chip, never in it; nothing
with it off or without truecolor. Snapshots unchanged. Feel review:
.scratch/render/tty/shadow/.

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