feat(storage): implement MDBX storage backend

[dokpark21]

Summary

Implement the foundational MDBX storage backend for CipherBFT per ADR-010. This PR adds the basic structure and skeleton
implementation using reth-db (MDBX) as the persistent storage layer.

Changes

New Files

• crates/storage/src/mdbx/mod.rs - Module definition and re-exports
• crates/storage/src/mdbx/database.rs - Database wrapper with configuration
• crates/storage/src/mdbx/tables.rs - Table key/value type definitions with Encode/Decode
• crates/storage/src/mdbx/provider.rs - MdbxDclStore implementing DclStore trait
• crates/storage/src/mdbx/wal.rs - MdbxWal implementing Wal trait
• crates/storage/MDBX_IMPLEMENTATION.md - Implementation status and TODO documentation

Modified Files

• Cargo.toml - Added reth-db workspace dependencies
• crates/storage/Cargo.toml - Added mdbx feature flag and dependencies
• crates/storage/src/lib.rs - Exposed mdbx module under feature flag

What's Implemented

• DatabaseConfig and Database wrapper for MDBX environment
• Table key types (CarTableKey, HashKey, HeightRoundKey) with proper encoding
• Stored value types with bincode serialization
• Skeleton MdbxDclStore with type conversion helpers between domain types and stored types
• Skeleton MdbxWal for write-ahead logging
• Feature-gated module (mdbx feature required)

What's NOT Implemented (Future Work)

• Actual MDBX read/write operations (currently returns Ok(None) or Ok(()))
• Table definitions using define_tables! macro
• Cursor-based range queries
• Transaction support (DclStoreTx)
• Pruning logic
• Crash recovery

Testing

Build without mdbx (default)

cargo check -p cipherbft-storage

Build with mdbx feature

cargo check -p cipherbft-storage --features mdbx

Run tests

cargo test -p cipherbft-storage

All existing tests pass. The mdbx module compiles successfully with the feature flag enabled.