feat: relayer atomic nonce mgmt


**Description**
Sync nonces across relayer.

**Problem Statement**
Nonce collision/replay.

**Technical Requirements**
- [ ] Nonce tracker service.

**Implementation Guide**
- **Suggested Approach:** Lock/Increment logic.
- **Affected Areas:** src/

**Acceptance Criteria**
- [ ] Nonce uniqueness guaranteed.

**Security & Audit Considerations**
- [ ] None.

**Definition of Done**
- [ ] Tested.