MCP-13: Add ConsensusBlock v1 wire codec

[aeyakovenko]

Summary

• add ledger/src/mcp_consensus_block.rs implementing ConsensusBlock v1 wire encode/decode
• enforce exact parser invariants:
  • version must be 1
  • aggregate/meta lengths are explicit u32
  • truncated and trailing-byte payloads are rejected
• add canonical signing preimage (signing_bytes) and leader sign/verify helpers
• preserve protocol policy that consensus_meta is opaque and authoritative for consensus-defined block_id
• add unit tests for roundtrip, signature verification, tamper detection, and parse rejection paths

Why this shape

• isolates consensus payload wire correctness in a small module
• minimizes diff by avoiding gossip/TPU/TVU pipeline changes in this issue

Tests

• cargo test -p solana-ledger --lib mcp_consensus_block::tests -- --nocapture

Fixes #15