proto: define MountRegister/MountList and MountPeer service
Adds the wire types for peer chunk sharing between weed mount clients:
* filer.proto: MountRegister / MountList RPCs so each mount can heartbeat
its peer-serve address into a filer-hosted registry, and refresh the
list of peers. Tiny payload; the filer stores only O(fleet_size) state.
* mount_peer.proto (new): ChunkAnnounce / ChunkLookup RPCs for the
mount-to-mount chunk directory. Each fid's directory entry lives on
an HRW-assigned mount; announces and lookups route to that mount.
No behavior yet — later PRs wire the RPCs into the filer and mount.
See design-weed-mount-peer-chunk-sharing.md for the full design.
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