#222 Replay: Add concurrent variable mutation handling

## Problem Multiple parallel branches modifying ctx.set_variable() simultaneously. Last write wins (ON CONFLICT DO UPDATE). ## Gap - No merge strategy for concurrent updates - Branch B can overwrite Branch A's variable - No conflict detection or warning ## Impact - Race conditions in parallel workflows - Non-deterministic variable state - Hard-to-debug data corruption ## Solution 1. Add optimistic locking (version counter) 2. Detect and warn on concurrent mutations 3. Optional: Implement merge strategies 4. Document parallel variable best practices ## Effort: Medium