[Parley] Conversation Simulator: Add sub-node coverage tracking #14950
Description
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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