graphics: state z=0 on a placement, so a mirror paints it over the text

kitty's default z is 0, which draws an image above the text, and that is
the value slosh has always relied on -- it places after the cell diff
precisely so the picture ends up on top. Saying nothing works because
the terminal supplies the default.

Anything mirroring the stream has no such default to fall back on. It
cannot tell "the emitter wants this over the text" from "the emitter
said nothing", and shellglass's viewer paints an unstated image under
the cells. That is fine until a cell over the picture carries an
explicit background -- and dim_unfocused gives one to every cell of a
pane at once, so the moment focus moved elsewhere the whole image
disappeared from the mirrored copy while the terminal, which took the
default, still showed it.

So the placement states z=0. Nothing changes on a terminal, since that
is the value it was already assuming; the mirror now paints the picture
over the cells and the dim pass runs underneath it, where it belongs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CRz3zwZJvBvzsR9k4cj6dd

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