feat: Job coordination for fallback share submission

[kwsantiago]

Summary

Adds job synchronization between DATUM Gateway and upstream pools to support fallback share submission when miners can't reach their gateway.

Closes #86

Changes

• Add job sync module for coordinating jobs with pool
• Add configuration options for job coordination
• Integrate job sync with protocol layer
• Add failover metadata to stratum subscribe response
• Update build system to include new module

Configuration

New options:

{
  "datum_enable_job_coordination": false,
  "datum_share_full_templates": false,
  "datum_allow_direct_failover": true
}

Status

This is an initial implementation (WIP) that requires significant work and comprehensive testing to complete.

Notes

• Feature is opt-in and backward compatible
• Requires corresponding pool-side implementation
• No mining firmware changes required initially