Parser playground

Low-level dev tool for the parser/interpreter, with the raw AST view. For the ball-flow ladder diagram, see the Explorer. Type an RC string below - the table shows a first-draft, provisional hand/limb assignment (see interpreter.js), and the validity line below it checks that the pattern conserves balls (every throw lands, nothing throws empty-handed) — still provisional, see the file's own doc comments for known simplifications.