feat(storage): add validated storage with WASM-based consensus (#10)

Add per-challenge storage system where validators must reach consensus
before data is accepted. This prevents abuse by requiring WASM-defined
validation logic and cryptographic signatures on all proposals and votes.

Core implementation in distributed-storage crate:
- ValidatedStorage: main struct managing proposals, votes, and commits
- StorageWriteProposal: represents a write request with value hash
- StorageWriteVote: validator vote with optional WASM validation result
- ConsensusResult: tracks consensus state and commitment status
- WasmStorageValidator trait: interface for WASM validation logic
- DefaultWasmValidator: passthrough implementation for testing
- ValidatedStorageConfig: configurable quorum size and timeouts

WASM runtime interface integration:
- ChallengeStorage: high-level API for WASM modules to access storage
- ChallengeStorageConfig: per-challenge storage configuration
- Full async support with proposal/vote/commit lifecycle

Key features:
- Configurable quorum size for consensus requirements
- Proposal expiration with automatic cleanup
- Duplicate and conflicting vote detection
- Cryptographic hashing of values (SHA256)
- Comprehensive test coverage for all operations

Dependencies:
- Added sha2 to wasm-runtime-interface for hashing

Author: echobt

Cargo.lock

@@ -93,6 +93,7 @@ pub mod replication;
 pub mod state_consensus;
 pub mod store;
 pub mod submission;
+pub mod validated_storage;
 pub mod weights;
 
 // Re-export main types for convenience
@@ -129,6 +130,13 @@ pub use state_consensus::{
     StateRootConsensus, StateRootConsensusError, StateRootProposal, StateRootVote,
 };
 
+// Validated storage with WASM consensus
+pub use validated_storage::{
+    ConsensusResult as ValidatedConsensusResult, DefaultWasmValidator, StorageWriteProposal,
+    StorageWriteVote, ValidatedStorage, ValidatedStorageConfig, ValidatedStorageError,
+    WasmStorageValidator, WasmValidationResult,
+};
+
 #[cfg(test)]
 mod integration_tests {
     use super::*;