feat(challenge-sdk): add P2P chain storage module

Complete distributed storage system for challenges with:
- Proposal/Acceptation protocol for data validation
- Custom validators per challenge (ProposalValidator trait)
- Consensus mechanism (2/3+ stake-weighted votes)
- Data categories: AgentSubmission, EvaluationResult, TaskResult, ConsensusScore, ExecutionLog
- Helper types: AgentSubmissionData, EvaluationResultData, TaskResultData, ConsensusScoreData
- P2P messages for sync, proposals, votes, consensus announcements
- ChainStorageState for managing pending proposals and committed data

Author: echobt

crates/challenge-sdk/src/lib.rs

@@ -67,6 +67,7 @@ pub mod database;
 pub mod distributed_storage;
 pub mod error;
 pub mod p2p;
+pub mod p2p_chain_storage;
 pub mod routes;
 pub mod storage_client;
 pub mod storage_schema;
@@ -83,6 +84,7 @@ pub use database::*;
 pub use distributed_storage::*;
 pub use error::*;
 pub use p2p::*;
+pub use p2p_chain_storage::*;
 pub use routes::*;
 // Note: storage_client and storage_schema have conflicting types with distributed_storage
 // Use explicit imports: platform_challenge_sdk::storage_schema::*