Validate Piles Notation

Parse and validate PILES (Puzzle Input Line Entry System) notation for specifying piece fusion groups in jigsawR. Covers syntax validation, parsing into group lists, plain-language explanation, adjacency verification against puzzle results, and round-trip serialization. Use when validating user-supplied PILES strings before passing to generate_puzzle(), debugging fusion group issues, explaining the notation to users, or testing round-trip parse/serialize fidelity.