[Parley] Conversation Simulator: Add sub-node coverage tracking

Summary

Add per-node coverage tracking for intermediate nodes in the Conversation Simulator, using the same X/Y format as root entries.

Current Behavior

• Root entries show coverage like "(2/3)" indicating visited/total replies under that entry
• Intermediate nodes (non-root entries) don't show their own coverage metrics

Desired Behavior

• All entry nodes should show coverage for their direct child replies
• Format: "(X/Y)" where X = visited replies, Y = total replies under that node
• This helps identify which specific branches haven't been tested

Context

Follow-up from Issue #478 - Conversation Simulator Sprint 1

Acceptance Criteria

• [ ] Each NPC entry displays coverage for its child replies
• [ ] Coverage updates in real-time as user navigates
• [ ] Visual indicator when a node's children are fully covered

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