Marching-squares contour extraction for 2D scalar grids.

Given a rows × cols grid of numeric values, segments/5 produces a set of line segments in data space where the scalar field equals each requested level. The renderer draws these segments as Contour series output — iso-lines suitable for topography, isobars, field intensity, level sets of optimization landscapes.

The algorithm:

• For each cell (a 2×2 square of grid values), classify its four corners as above or below the target level.
• Each classification gives a case index 0..15; the case tells us how many segments (0, 1, or 2) to emit through the cell and where they enter / exit on the cell boundary.
• Each segment endpoint is linearly interpolated along the cell edge where the scalar value crosses the level.

The 16-case table is unambiguous for 14 cases; the two saddle cases (5 and 10) are broken by the "mean of four corners" convention — adequate for typical scientific data.

Output: [{level, [{p1, p2}, {p3, p4}, ...]}, ...] where each p is {x, y} in data space.