cbor: rename test_cbor_rw to test_binary, drop explicit modules stanza
- Rename test/test_cbor_rw.{ml,mli} to test/test_binary.{ml,mli} to
match the lib/binary.ml rename (E605).
- Drop the (modules ...) field from fuzz/dune (E523).
12 Obj.magic usages in cbor.ml remain — they encode the record codec's
CPS continuation walk (extracting names/decoders without having a value
in hand). A refactor to explicit field-list + Dict-based assembly is
tracked as follow-up work; the Obj.magic calls are isolated to codec
construction and never touched at runtime.