## Summary
Adds `dev-docs/pathfinding.md` — a reference for how creature pathfinding works in ModernUO, written so a future contributor (human or AI) can reason about it without re-deriving it from the code.
Covers:
- **The stack** end to end: `ApproachTarget` → `PathFollower` → `MovementPath` → `BitmapAStarAlgorithm` → `StepCache` → `MovementImpl` slow path (and that the removed FastAStar survives as the slow path).
- **Windowed-A* limits** (`AreaSize=38`, `MaxSearchNodes`, Z planes) and what they mean (2D-adjacent-but-obstacle-separated / a-floor-up goals are unsolvable by design).
- **StepCache**: second-touch warming, LRU-bounded memory, lazy `.swb` backing stores — with **measured** disk sizes (~565 MB/Trammel; ~1.5–2 GB all facets).
- **Four config levers** in a table: `pathfinding.enable`, `bitmap_pathfinding_cache`, `pathfinding.maxResidentChunks`, `pathfinding.maxSearchNodes`.
- **Small/crappy-hardware shard spectrum** (cache off ≈ FastAStar at ~1× with zero warming memory, up through baked `.swb`).
- **Diagnostics & tooling** (`[PathCacheStats`/`[PathRecord`/`[PathBake`, the MapDump tool, the benchmark suite) and the Debug/Release test note.
- **Future work**: background-thread bake, long-traverse BDN scenario, swim `SourceZ` bake, and the `.swb` size-reduction roadmap.
## Note on scope
This is docs-only. It documents the *complete* pathfinding system, so it references a couple of pieces that ride in separate PRs (the `ApproachTarget` AI fix and the `pathfinding.maxSearchNodes` setting). If those havent merged yet, sequence this after them so the doc doesnt describe unshipped code. The StepCache/`.swb`/provider material it documents is already on `main`.
978f314f0e