Function sliceGizmoGeometry

Remarks

Three things the previous single-quad visual got wrong about the operation it depicts:

• The cut face had a hard top and bottom. The clip volume is the slab between two parallel planes bounded by two side walls, and is not closed off along the face's own vertical axis — buildings are cut floor-to-roof. Drawing a bright rim all the way round said the opposite: that the pane you see is where the cut stops. The face now fades out toward both ends and is outlined only on the two SIDE edges, which are the boundaries that genuinely exist.
• Its height was capped at 300 m, so a campus-scale slice got a card a fraction of its own width and the cut visibly ran on past it. The extent now scales with the slice.
• The volume showed only as a flat ground rectangle. Adding the back wall's two edges closes the picture into something with depth: front face, ground footprint, back edges.

The back rails sit on the TRUE back-wall edge (−n · depthM ± u · halfWidth, running along the face's own axis), while the footprint stays the ground projection it has always been. For an untilted plane — the default, and what place resets to — the two coincide exactly; under tilt the offset between them is the tilt, shown rather than hidden.