fix: cargo standability and clear old task target

- Make is_standable return true for cargo tiles (logs, rocks can be stood on)
- Clear ambulatory.target when assigning new task to prevent old targets persisting

These fixes address the 'Lost Signal' bug where dorfs would freeze:
1. Cargo tiles were incorrectly flagged as non-standable, blocking pathfinding
2. Old Idle task targets persisted when new jobs assigned, causing dorfs to path to wrong locations

src/entities/tasks/job_pathfinding.rs

@@ -301,6 +301,7 @@ pub fn job_pathfinding_system(
                     *state = TaskState::Pending;
                     ambulatory.current_path = None;
                     ambulatory.path_index = 0;
+                    ambulatory.target = None; // Clear old target from previous task
 
                     info!(
                         "[PATHFIND] SUCCESS: Assigned job {:?} to dorf {:?} with approach {:?}",