#220 Replay: Add nested parallel branch replay support

## Problem ReplayContext doesn't re-execute parallel_fork operator. Parallel branches shown in audit log but not re-executed in simulation. ## Current State - Parallel branches tracked in audit log - ReplayContext has no parallel handling - Only sequential replay works ## Impact - Complex parallel workflows hard to debug - Cannot validate parallel execution paths - Missing visibility into branch interactions ## Solution 1. Add recursive replay context for parallel branches 2. Track branch execution order in simulation 3. Validate branch results match original ## Effort: Medium