feat(p2p-03): add p2p config surface (#671)

[jlon]

Summary

• Add client and master config schema for P2P settings and runtime policy inputs.
• Keep P2P disabled by default so this PR does not change runtime behavior.

Design

• Group client knobs under client.p2p and keep master policy state in config, not code paths.
• Parse byte and duration strings during init and reject invalid combinations early.

Implementation

• Add ClientP2pConf, default cache-dir resolution, validation, and client init wiring.
• Add master-side policy version, signing key, and whitelist fields with defaults.

Verification

• cargo test -p curvine-common p2p_default_cache_dir_prefers_xdg_cache_home
• cargo test -p curvine-common p2p_init_rewrites_legacy_testing_cache_dir
• cargo test -p curvine-common cluster_conf_allows_minimal_p2p_block

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