feat(p2p-04): add client p2p module shell (#671)

[jlon]

Summary

• Create the client p2p namespace and base discovery/transfer types.
• Keep the module as compile-time scaffolding only, with no read-path or service behavior.

Design

• Limit this PR to DTOs and snapshots that later P2P layers can reuse.
• Make ChunkId file/version aware so later cache and fetch work does not need another key rewrite.

Implementation

• Export curvine_client::p2p from the crate root.
• Add discovery snapshots plus transfer request/response and chunk identity types.

Verification

• cargo check -p curvine-client
• cargo test -p curvine-client --lib

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