Proxy-Driven Server for Flexible Files Version 2

Parallel NFS (pNFS) with the Flexible Files Version 2 layout type supports client-side erasure coding and per-chunk repair between clients and data servers. This document extends that architecture with a proxy server (PS) role: a registered peer of the metadata server that polls the metadata server for work assignments and carries them out -- moving a file from one layout to another, reconstructing a whole file from surviving shards, or translating between codecs for clients that cannot participate in the file's native encoding (including NFSv3 clients). All PS-MDS coordination is fore-channel: the metadata server returns work assignments inline in the response to a PS-initiated PROXY_PROGRESS poll, and the PS reports completion via a fore-channel PROXY_DONE. No callback operations are required for the PS protocol.