Nonce management for transaction submission

[tac0turtle]

Summary

Implement local nonce (sequence number) tracking and correction for blob transaction submission.

Context

Parent: #4

Cosmos chains use account sequence numbers as nonces. Correct nonce management is critical — wrong sequence = rejected tx.

celestia-node's approach (query once at startup, increment locally, regex-parse error strings on mismatch) works but is fragile. We should rebuild this simpler and more robust.

Design

Core behavior

1. Startup: Query on-chain account sequence via auth.QueryAccount gRPC
2. On broadcast: Use local sequence, increment optimistically
3. On success: Keep incremented value
4. On ErrWrongSequence: Re-query on-chain sequence (don't regex-parse error strings), re-sign, retry (max 3 attempts)
5. On other failure: Do not increment, return error to caller

Thread safety

• Mutex around sequence read/increment/broadcast for single-account use
• Per-account mutex when multi-account is enabled (Multi-account support for transaction submission #5)

Interface

type NonceManager interface {
    // NextSequence returns the next sequence number for the given address.
    // Blocks if another tx is in-flight for this address.
    NextSequence(ctx context.Context, address string) (uint64, error)

    // Commit confirms the sequence was used successfully.
    Commit(address string)

    // Rollback indicates the sequence was not used (tx failed).
    Rollback(address string)

    // Reset re-queries the on-chain sequence for the given address.
    Reset(ctx context.Context, address string) error
}

Error handling

• Check ABCI error codes directly where possible, not error string patterns
• Wrap errors with context: address, expected sequence, actual sequence, attempt number

Non-goals

• Parallel nonce assignment for the same account (handled by Multi-account support for transaction submission #5 multi-account instead)
• Speculative nonce gaps (always sequential)