New library/packet/fork-leaders.js: a pure, deterministic router for one
paired-row leader, forking a stem at a 45° chamfer into two short legs
(one per pin), in row and trunk modes. All segments H/V/45°. Returns
polylines for drawing and a deduped segment list (stem once) so a fork
counts as one polyline for crossings, plus octilinear and through-pin
reports. Imitates the poster-render octilinear lane routing (read, not
called). Adds check:fork-leaders (both modes, 0 crossings on a 220-row
monotone set, through-pin + poison arms).