feat(pathfinding): implement Phase 3 reactive pathing

- Add validation_cooldown field to Ambulatory component
- Implement validate_next_steps to check walkability of next 3 path nodes
- Integrate validation into movement system (every 10 frames)
- Trigger re-path when validation fails (path blocked or invalid)
- Entities now detect and recover from invalid paths automatically

src/entities/shared_systems/pathfinding.rs

@@ -502,6 +502,17 @@ pub fn movement(mut query: Query<(&mut Ambulatory, &mut Transform)>, tilemap: Re
                 return;
             }
 
+            if ambulatory.validation_cooldown > 0 {
+                ambulatory.validation_cooldown -= 1;
+            } else if let Some(path) = &ambulatory.current_path {
+                if !validate_next_steps(&tilemap, path, ambulatory.path_index, 3) {
+                    ambulatory.current_path = None;
+                    ambulatory.validation_cooldown = 10;
+                    return;
+                }
+                ambulatory.validation_cooldown = 10;
+            }
+
             if ambulatory.walk_speed > 0. {
                 if ambulatory.step_recovery <= ambulatory.walk_speed as u32 {
                     ambulatory.step_recovery += 1;
@@ -534,6 +545,17 @@ pub fn movement(mut query: Query<(&mut Ambulatory, &mut Transform)>, tilemap: Re
         });
 }
 
+fn validate_next_steps(tilemap: &TileMap, path: &[Vec3], start_index: usize, steps: usize) -> bool {
+    let end = (start_index + steps).min(path.len());
+    for i in start_index..end {
+        let pos = path[i].as_ivec3();
+        if !is_standable_tile(tilemap, pos) {
+            return false;
+        }
+    }
+    true
+}
+
 fn is_standable_tile(tilemap: &TileMap, pos: IVec3) -> bool {
     let can_stand_in_tile = tilemap
         .floor_tiles