feat(dupfind): add show command, normalize tests, and structural hashing
- Add `dupfind show HASH` to display normalized AST for a hash cluster
- Add pp_expr pretty-printer for the internal AST
- Replace Marshal-based hashing with structural tag-length-value encoding
- Add expr_size and sub_hashes for fingerprinting support
- Add normalize unit tests for alpha-equivalence verification
- Add missing .mli files for test modules