Make the plane solid, and take the posed scene back out

The cone is fully opaque now: solid colour, deep blue at the edges and near-white
down the core, dimming as it runs away. Depth is carried by BRIGHTNESS instead of
by alpha, so it still reads as receding without anything showing through.

That settles an argument the translucent version could not win. Its opacity and
how well you could tell above-the-plane from below-the-plane were the same dial
pulling opposite ways: readable enough to show the split and it veiled the whole
cone, subtle enough to look right and there was nothing to see. Solid, a body
above the plane is drawn on top of it and a body below is hidden behind it. The
trade is that you cannot see the crowd through it, which is the thing to judge on
the phone.

The `alpha` parameter went with it rather than being left in the signature lying
about what it does.

laserScene is gone. T.pause(true) stays -- it freezes the world but keeps drawing,
which is worth having whenever a still is what you need to look at.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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