Implement LxrMiningSignature Type

Summary

Implement the core LxrMiningSignature type as defined in the specification.

Background

The LxrMiningSignature is a new signature type that requires proof-of-work to create, enabling mining-based participation in the Accumulate network.

Requirements

Define LxrMiningSignature structure in protocol/signatures.yml
Implement signature interface methods
Include mining account field for namespace isolation
Implement hash computation with LXRHash

Implementation Details

type LxrMiningSignature struct {
    // Mining proof fields
    MinerADI        *url.URL
    Nonce           uint64
    CandidateHash   [32]byte
    
    // Context fields
    MiningAccount   *url.URL  // App namespace
    TransactionData []byte
    BlockHash       [32]byte
    Timestamp       uint64
    
    // Configuration
    BaselineTarget  [32]byte
    TableSize       uint32
    
    // Standard signature fields
    PublicKey       []byte
    Signature       []byte
}

Acceptance Criteria

Signature type defined in YAML schema
Generated code includes new type
Interface methods implemented (Verify, Hash, Metadata, etc.)
Mining hash calculation includes MiningAccount for isolation
Unit tests for signature verification
Integration with existing signature validation

Dependencies

Depends on: #3666 (KeyPage Schema)

Epic: &37
Specification: 32-lxr-mining.md