v6.x backlog: async client (clappform.aio)

[Pandys]

Deliberately out of v1: primary persona is notebooks, where a sync API wins. Transport is isolated so this is additive.

• clappform.aio.Clappform mirroring the sync surface on grpc.aio
• Generated layer: emit aio variants from the same protoc-gen-clappform templates
• The hand-written ergonomic layer (DataFrame flows, pagination, retries) is the real cost — it doubles; only worth it on concrete demand