- Add AvgDifficulty and DiffCurrent fields to PerformanceMetrics
- Calculate avg difficulty as HashesTotal/SharesGood in XMRig stats
- Add difficulty data to TT-Miner stats using pool difficulty
- Display "Avg Diff" stat in stats panel with k/M/G/T formatting
- Add WriteStdin to MockMiner for test interface compliance
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>