fix: four interlocking pathfinding bugs causing panic, failure, and yo-yo teleportation

1. CSV Overflow Panic: Use saturating_sub to prevent underflow when total_failed_paths > n
2. Heuristic Scale Mismatch: octile_distance_3d now divides by ITILE_SIZE to match g-score units
3. Goal Z Offset: Correct target.as_ivec3() - ivec3(0,0,1) not ITILE_SIZE (target already has +1.0)
4. Gravity Yo-Yo: Clear current_path and target when entity falls to prevent teleportation loop

src/entities/shared_systems/pathfinding.rs

@@ -183,8 +183,8 @@ pub fn prepare_paths(
             continue;
         };
         let start = transform.translation.as_ivec3();
-        let goal = target.as_ivec3() - ivec3(0, 0, ITILE_SIZE);
-        let distance = octile_distance_3d(start, goal) / ITILE_SIZE;
+        let goal = target.as_ivec3() - ivec3(0, 0, 1);
+        let distance = octile_distance_3d(start, goal);
 
         if distance <= PATHFINDER_SHORT_PATH_MAX_TILES {
             let path = calculate_path_benchmarked(&tilemap, start, goal);
@@ -352,6 +352,8 @@ pub fn movement(mut query: Query<(&mut Ambulatory, &mut Transform)>, tilemap: Re
             let current_pos = transform.translation;
             if !is_standable_tile(&tilemap, current_pos.as_ivec3()) {
                 transform.translation.z -= TILE_SIZE;
+                ambulatory.current_path = None;
+                ambulatory.target = None;
                 return;
             }
 
@@ -433,9 +435,9 @@ fn calculate_movement_cost(move_dir: IVec3) -> i32 {
 }
 
 fn octile_distance_3d(a: IVec3, b: IVec3) -> i32 {
-    let dx = (a.x - b.x).abs();
-    let dy = (a.y - b.y).abs();
-    let dz = (a.z - b.z).abs();
+    let dx = (a.x - b.x).abs() / ITILE_SIZE;
+    let dy = (a.y - b.y).abs() / ITILE_SIZE;
+    let dz = (a.z - b.z).abs() / ITILE_SIZE;
     let (dmax, dmid, dmin) = sorted_desc(dx, dy, dz);
     10 * dmax + 4 * dmid + dmin
 }
@@ -767,7 +769,7 @@ fn write_benchmark_csv(bench: &PathfindingBenchmark, filename: &str) -> std::io:
         let duration = bench.path_calc_times_us.get(i).copied().unwrap_or(0);
         let length = bench.path_lengths.get(i).copied().unwrap_or(0);
         let nodes = bench.nodes_expanded.get(i).copied().unwrap_or(0);
-        let success = i < (n - bench.total_failed_paths as usize);
+        let success = i < n.saturating_sub(bench.total_failed_paths as usize);
         writeln!(file, "{},{},{},{},{}", i, duration, length, nodes, success)?;
     }