mount: tier-2 chunk directory + FetchChunk streaming on one gRPC port
Collapses the old two-port design (HTTP peer-serve + separate gRPC
directory) into a single gRPC service that handles every mount-to-
mount exchange: ChunkAnnounce, ChunkLookup, and the new FetchChunk
byte stream.
* peer_directory.go: fid -> holders shard, HRW-gated; returns holders
in LRU order; capacity-bounded; Sweep handles eviction under
write-lock while Lookup runs under RLock (hot path is concurrent).
* peer_grpc.go: single MountPeer gRPC server implementing all three
RPCs. FetchChunk frames bytes at 1 MiB per Send so the default
4 MiB message cap does not constrain chunk size; cache miss
returns gRPC NOT_FOUND so clients distinguish miss from transport
error. Reuses pb.NewGrpcServer for consistent keepalive + msg-size
tuning.
* peer_bytepool.go: sync.Pool wrapper around *[]byte that the server
uses to avoid a fresh 8 MiB allocation per FetchChunk call.
* WFS wiring starts the gRPC server on option.PeerListen (the single
peer port) using the advertise address resolved in PR #3 as the
HRW identity. A background sweeper evicts expired directory
entries every 60 s.
