Implements a TOML-based dialogue tree system for NPCs with:
- DialogueChoice: player response options with optional conditions
- DialogueNode: NPC text with choices and optional actions
- DialogueTree: complete tree with root node and node graph
- Validation for root_node and next_node references
- load_dialogue() for single files, load_all_dialogues() for directories
Includes librarian dialogue example with nested conversation flow.
