Billiards in right triangles: code deposit for four papers

Exact-arithmetic code supporting four papers on billiards in right triangles: The orphan theorem; The necklace of a rational right triangle; No arrivals; and Non-perpendicular periodic orbits in mirror directions of right triangles. A small core library (lib/) and one directory per paper holding one script per statement; MANIFEST.md maps each script to the statement it supports.