feat(p2p-01): add file version epoch proto foundation (#671)

[jlon]

Summary

• Add version_epoch to shared file metadata used by later P2P read identity.
• Keep legacy FileStatus payloads decodable with a default epoch of 0.

Design

• Extend the existing FileStatus path instead of introducing a parallel P2P-only message.
• Seed inode-side version state now so later cache and transfer keys can stay version-safe.

Implementation

• Update common.proto, FileStatus, proto conversions, and inode view/file mappings.
• Add proto compatibility coverage for round-trip and legacy decode behavior.

Verification

• cargo test -p curvine-common file_status_proto_roundtrip_keeps_version_epoch
• cargo test -p curvine-common file_status_proto_decodes_legacy_payload_without_version_epoch

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