feat(p2p-02): add master policy protocol primitives (#671)

[jlon]

Summary

• Add wire-level P2P policy fields and a dedicated GetP2pPolicy response shape.
• Add signing helpers without introducing client sync or runtime policy application yet.

Design

• Keep backward compatibility by extending existing master info payloads with defaulted fields.
• Use stable HMAC input framing and accept comma-separated transition signatures for key rotation.

Implementation

• Extend master proto/state/proto-utils mappings with policy version and allowlists.
• Add sign_p2p_policy and signature verification helpers plus compatibility tests.

Verification

• cargo test -p curvine-common p2p_policy_signature_roundtrip_verifies
• cargo test -p curvine-common p2p_policy_signature_decoder_accepts_transition_list
• cargo test -p curvine-common master_info_proto_decodes_legacy_payload_without_p2p_policy_version

[jlon]

Part 2/17 of the split P2P series that supersedes #671.

This PR targets , so the visible diff is cumulative. Review the slice described in this PR body instead of treating the whole diff as new.

Intended review delta is p2p-02 on top of #762.

[jlon]

Review note: this is part 2/17 of the split P2P series that supersedes #671.

This PR targets main, so the visible diff is cumulative. Please review the slice described in this PR body.

Review after #762.