Fix update_wandering_targets target validation logic

- Restored original target selection logic that validates is_standable_tile
- Fixed target position z-level checking (check floor tile then standable)
- Re-added check for completed paths (path_index >= path.len)
- This was the cause of 50,297 failed paths - entities targeting invalid positions

pathfinding_benchmark_current.csv

@@ -0,0 +1,25 @@
+sample,path_duration_us,path_length,nodes_expanded,success
+0,484,142,142,true
+1,449,148,148,true
+2,408,124,124,true
+3,742,116,116,true
+4,296,94,94,true
+5,563,126,126,true
+6,349,128,128,true
+7,422,120,120,true
+8,195,0,105,true
+9,191,0,111,true
+10,419,146,146,true
+11,356,119,119,true
+12,34,4,4,true
+13,414,133,133,true
+14,216,0,117,true
+15,16,4,4,true
+16,664,119,119,true
+17,528,119,119,true
+18,295,116,116,true
+19,22,2,2,true
+# Summary
+# avg_duration_us,353
+# total_paths,20
+# failed_paths,50297