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 {:?}",

src/world/tiles/tilemap.rs

@@ -309,6 +309,11 @@ impl TileMap {
     /// O(1) standability check using bit-packed chunk data.
     /// Returns false if chunk is not loaded (unloaded chunks have no valid tiles).
     pub fn is_standable(&self, world_pos: IVec3) -> bool {
+        // Cargo tiles override chunk standability - dorfs can stand on cargo to pick it up.
+        if self.cargo_tiles.contains_key(&world_pos) {
+            return true;
+        }
+
         let chunk_pos = world_to_chunk(world_pos);
         let Some(chunk) = self.chunks.get(&chunk_pos) else {
             return false;