chain: flatten request-batch's dead outer chain-error or record the escape hatch

[mfw78]

Problem. request_batch's outer chain-error is structurally dead on the reference host: each entry already carries its own rpc-result, so the outer error is an unused escape hatch.

Change. Either flatten wit/nexum-host/chain.wit request-batch to list<rpc-result>, or record in ADR-0011 why the outer error is retained for other host implementations.

Relates. #206. Surfaced by the #219 review.