refactor: simplify to time-sliced synchronous path queue

The async pathfinding with StandableBitGrid caused massive lag due to synchronous
bounding box calculation in the main thread (millions of hashmap lookups for long paths).
Additionally, the splicing logic had a catastrophic bug where it applied a single
finished path to ALL entities unconditionally.

Solution:
- Revert to thread-local synchronous A*
- Implement PathRequestQueue to process max 8 paths per frame
- Keep provisional paths for immediate movement
- Since entity walks provisional path, full path calculates from current position,
  eliminating the need for complex splicing logic.

src/entities/shared_systems/mod.rs

@@ -1,2 +1 @@
-pub mod async_pathfinding;
 pub mod pathfinding;