feat(p2p-05): add fetched cache backend (#671)

[jlon]

Summary

• Add the fetched-cache backend needed for later P2P read reuse.
• Keep the backend dormant until a later PR wires it into the read path.

Design

• Separate normal file-backed cache entries from append-only fetched-log entries.
• Track checksum, mtime, eviction, and corruption state so later read-path work has a stable backend contract.

Implementation

• Add CacheManager with TTL, sharding, batch append, checksum validation, and snapshot accounting.
• Export the backend from p2p and add the ring dependency for digest support.

Verification

• cargo test -p curvine-client duplicate_put_refreshes_without_rewrite
• cargo test -p curvine-client fetched_batch_put_writes_and_reads_back
• cargo test -p curvine-client wrong_checksum_hint_is_detected_on_read

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