[Mehulsri07]

A 3D slice on the Section screen: switch the drawing from "Flat drawing" to "3D slice" and the section becomes a block cut out of the ground, like a slice of cake.

• Front: the section itself, exactly as on the flat drawing. Back: a cut parallel to the line, behind it (250 m to 2 km, 500 m to start). Ends: cuts across at A and A′. Top: the strip of ground with the borewells inside it as pins, and the line along its front edge.
• One flat side per straight part of the line; the back follows the line with mitred corners, and never folds over behind a short part after a bend.
• Every side is drawn like the flat drawing (faded = likely, paler = rough guess, hatched = not enough data; borewells solid), all at one scale so layers meet at the edges. Only outward sides are shown, so the inside is never seen.
• Drag to turn and tilt, or use the arrow keys; Reset view. Heights start stretched so the block is about half as tall as it is long, with a slider from true scale upwards.
• Built from flat SVG panels with CSS 3D transforms; no new libraries.

Checks: 140 unit tests (left offset, mitred and capped corners, no fold-over after short parts, faces and their direction, pins only behind the line, one height range); the real-app test switches to the 3D slice; screenshots checked for a straight and a bent line from two angles.

Review fixes: the back folded over where a short part followed a bend inwards; the block was far too tall on short lines; the arrow keys tilted the opposite way to dragging; the turnable view is announced as a 3D view with its instructions.

Not done: map tiles on the top (it is plain land colour); a borewell just past a bend can show on both neighbouring sides.