Add pathfinding A* system (disabled, in development)

- src/pathfinding.h: A* on navmesh triangles, funnel algorithm, Papyrus movement
- docs/: research documents for pathfinding and navmesh API
- Not yet integrated (include commented out in plugin.cpp)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Pyrhame