Function io-copypub

io-copy — copy bytes from any Read source to any Write Bytes sink until end-of-stream on the source, returning the total bytes copied.

The real implementation needs capability-handle impls of Read and Write so the effect row can be resolved; until those land, the body here panics. The signature is the spec signature with the existential (Seq U8 _) replaced by Bytes (see divergence 1) and the effect variable elided because no constraint can mention the Read/Write impls' effect rows yet.